I use mailwasher and when you need to register (or at least this was the case before the owner went money-mad and actually started charging) he asks for a donation.
The default choice is $10 but this can be lowered to $3 or raised to $50. That way, it felt like a donation rather than a flat purchase.
Perhaps you could to the same? Minimum donation $90 (for us poor sods who push trolleys for a living on sub-mimimum wage), standard donation of $100 (what you expect) but can go as high as $200 (not gonna happen but you never know).
It's like voluntary price-skimming (business people will know this term) and it could generate some extra cash
