Sine for ppl from USA 20$ is not much, for ppl from e.g. Eastern Europe it is not not much, maybe intruduce some different registrations:
BRONZE - 5$
SILVER - 10$
GOLD - 15$
Platinum - 20$
Diamond - 50$
with ability to upgrade by paying more money.
Also, I suggested this 2+ times, please consider Donor List, some people will pay not only to support 7-zip/7-max but also to be on such list (google will catch them on such list).
7zip is Open Source.