Why are people on this forum so antagonistic? If they made this kid rake leaves or mow the lawn or something I'd get it, but putting this weird financial burden on a kid is a different story. If he's too young to understand how to do the bills (and I can see legitimacy in that - after all, he has dyslexia) then they're giving him a responsibility that is more than mechanical, you feel? Like he probably knows if he messes up then he and his family will be in some stuff, whereas you can't really mess up your living situation by raking leaves incorrectly.
OP, provided your learning disability doesn't get in the way, I feel like paying the bills could become really automatic and simple for you, and it would effectively be a chore that your parents are making you do. That is, unless you've been doing this for a while already and you still are having trouble. If so, you could always just do a horrible job on purpose so they don't make you do the bills anymore.
How does this process work? You're doing the bills for a store? Does that mean you're putting in orders for inventory or something? Are you digging through receipts or something?