Here's an update in case anyone else is thinking of ordering from BTL! After a lot of online comparison shopping, I decided to order both liner and underlayment from btl. I ordered the BTL24 for the liner and 8 oz geotextile underlayment.
Pros: The liner is very thick and tough. They had previously sent me a sample of the BTL40, which is even thicker and tougher, however it's quite inflexible, and I couldn't justify that I needed this level of thickness compared to their thinner (and much cheaper) options. I decided to go with the 24 because they advised that regardless of which liner I got, I would need to cover all exposed liner to protect from sunlight if I wanted it to last a long time (either with water, dirt, or something else). Due to the way the existing concrete is slanted on the sides and not very evenly poured (one end of the pond is much higher than the other) I knew there was going to be exposure that would have be covered no matter what kind of liner I used, and sunlight seems to be the main liner enemy here.
The 24 was not at all thin though, it's only slightly more flexible than the 40. It was sort of like folding cardboard over the pond. I think it might be difficult to conform to a more complicated shape of pond as small as mine, as it was I have only one major fold, but the pond is sort of a shallow scooped out shape with no shelves or anything, the sharpest turn required was at the wall at the lower end. There are several wrinkles in the bottom (including along where the liner was folded before) which aren't budging as it's quite stiff. Incidentally, there were several other retailers that also seem to re-sell reinforced polyethylene pond liners from BTL, and recommendations on how to use it varied depending on the retailer. While BTL themselves recommended the very thick more expensive option, and couldn't really tell me the practical difference between the different versions of liner (or maybe I wasn't asking the right questions in terms of finding out which liner would be sufficient/what would puncture it), other sellers seemed to recommend the thinner ones unless the pond was very big because of the difficulty of install and the fact that even the thinner ones are very puncture resistant. Having gotten the 24 however, I'm satisfied that nothing, including dog claws which I was concerned about, is going through it anytime soon, especially with it being laid over concrete. I had also inquired about patching if it did get a hole, and they included a freebie section of patching tape with the order (which I have not yet had to use).
They sold a competitively priced 8 oz underlay that was very thick (they also sent a sample of this). It beat the price and thickness of any other underlay I could find, and they made both in custom sizes. Since my pond has kind of funny dimensions (17x26) I would have ended up ordering a much larger than necessary liner and underlay if I'd had to go with non-custom sizes from another site. This was another factor in choosing this type of liner over the rubber, with EPDM I would have had to go up to such large size the cost difference was major, while from BTL I could order the size I actually needed.
Now the cons...
The biggest con was a problem with the underlay... though custom sized and very thick, cushy, and feeling durable, it had a seam down the middle which was melted together to join two pieces, and where it was joined it had numerous large holes from being melted together! Since it was meant to protect the liner from the rough concrete, having holes I could easily put my thumb through right through to the concrete was a big problem (especially since I ordered it in custom size so there was nothing to block the holes with). I ended up having to cut a long strip of overlay of a different kind which I'd ordered for another purpose in order to cover the entire seam so the concrete did not rub up against the liner... which kind of defeated the purpose of the custom sizing. I had purchased a different, thinner underlay from another site to go over the exposed edges of the liner (which ended up short since I had to use some of it to patch the other one). I found that this thinner underlay also does not wick water the way the BTL underlay does... the BTL underlay should definitely not be used as an overlayment for that reason: it wicks water heavily so it should never contact the pond water.
The liner, while tough and only 17x26, had 3 seams. After seeing the underlay seam issue, I was a little concerned that these 3 seams on what is not a very large liner will be potential areas of failure, much more so than anything puncturing the liner. I would have been much happier if it hadn't had so many seams (or better yet, none). However, there are no leaks so far so maybe the liner seams are better made than the underlayment seam.
The other con was the inconvenience of ordering from them. There is an online order form, so I assumed this would be possible before I'd contacted them by email to discuss which type of liner I should go with (other pond liner sites that did not have online order forms at all just got crossed off my list from the get-go). The order form gives a price estimate for the liner based on material and dimensions, but it doesn't include shipping (which is costly since it's based on weight... though I was only one state away). If you fill out a full order including your location, instead of giving shipping costs, it asks you to submit the form and says they will contact you to confirm how much the shipping will be. I did not do that but continued my email discussion to get exact estimates since I'd already contacted them by email at that point. (On the very smallest dimensions shipping is calculated automatically based on zip code, but not past a certain size/weight). The underlayment cannot be estimated for cost at all online (neither the basic price by dimension nor the shipping), you have to contact them. Then you have to call on the phone during business hours and pay by card over the phone (when I asked about paypal or online payment options, they said the alternative to the phone was mailing a check... I get the impression they are used to doing business in person the 'old fashioned' way). Email estimates also didn't include tax, so factor that in as well if your state has sales tax. This took a while of back and forth, and remeasuring and asking about the price and shipping costs of different types of liner and the underlayment. After I gave them my card information over the phone, it is charged right away but there's no email confirmation or receipt until the order ships, over a week later. Total turnaround time from paying over the phone to liner received was 9 days; shipping was fast once it took place, but the interim was long and slightly nerve wracking, maybe as I've come to expect some sort of receipt once the money comes out. Now, for those people who don't mind calling stores and getting personal service and ordering over the phone, the need to do so will probably not be a con at all, but I prefer to do it all online at any hour of the night
, especially if I'm comparing prices and mulling over decisions. I like to see it all in advance: the shipping options, how long it will take and how much, total costs for different versions of the liner, is it cheaper to get the underlay from a different site, etc. until I'm good and ready to hit pay; and then get an email receipt right away and an idea of when the liner is going to arrive.
For what it's worth, they said they might revise the site to allow ordering the underlay online in the future (and hopefully, the shipping calculator as well, most sites have these during the order process). That would be a good business move, I think, and probably encourage more purchases as the process is a bit clunky compared to other sites.