make a minimum donation amount (1 dollar)
and if they donate again, just add it to their pool, with the same number
for example:
Guy 1 donates 20USD (067)
Guy 2 donates 10USD (089)
Guy 1 donates 05USD (067)
so in the end really
Guy 1 donates 25USD (067)
Guy 2 donates 10USD (089)
the...