I've been dealing with f/t for a long time and this is what I've found out. Anything without hair is good for about 6 months, as long as they're sealed good with minimal air(certain people sell their stuff vacuum sealed). I've had stuff with hair last up to a year. So a bag of rat pups will last you awhile, just make sure that you seal them really good and get as much air as you can out.
It can, but it most likely doesn't unless that snake wasn't fed properly. 15% of 140 grams is like 21 grams. That snake should be on rat pups at that weight. With that in mind that 10-15% rule barely applies and it's more like a 15% rule in this case. Also pet smart f/t stuff is like $8.