I got a referral through my gender clinic for mine which included some really nice storage prices.
Abstinence is definitely encouraged and seems to help. They should let you know if you've got enough stored to have a good chance at a future pregnancy, but you can likely just go for a second time to store more without increasing the costs, at least it didn't increase my costs.
My first appointment resulted in 11 straws, and from memory each storage straw is one try. By the second appointment I had 25 or so, and thankfully they were much more healthy that time, but that amount is kinda overkill from what they described.
Personally I wouldn't want to do mail in, just in case. It's insanely awkward going to the lab, but they really prefer you do it there to have the best chance of viable sperm to store. But mail in might be okay, I never researched it because it wasn't an option for my place.