There's nothing wrong with open molds as long as the underside is not going to be seen and they are filled consistently without bubbling. I generally don't cast like that because under vacuum, it would be a nightmare for me. If if looks good, by all means, more power to him.

In terms of price, I think 14 pieces for $24 shipped is a heck of a deal.

Keep in mind, the materials aren't cheap, I just spent about $400 on resin and silicone in the past two weeks. Just by eyeballing what you dumped out of that bag, my guess it's about 200-250ml. Going with 200, that means less than 20 set per gallon. The resin I use is a little over a $100 a gallon shipped. So, you're looking about $5 a set and that doesn't include the silicone for the mold (stuff I use is over $200 per gallon kit shipped).

If you think of it another way, ask yourself how long would it take you to make your own tarp (or 14 of them), then ask if $24 is worth your time.