Hello Zimbra!
Our service works just like submitting a bid directly on eBay except we submit the bid for you in the final seconds of an auction. So, yes, if your Max Bid is $100.00 that is what we submit to eBay when we submit your bid - eBay's proxy system handles the rest.
Since the system is set up to use only the amount of your bid that is required to make the next bid increment, if all that is needed of your bid is $20.51, that is what your bid would be for. (note that your example does not take bid increments into account, if you are unfamiliar with the bid increments on eBay, please check here). However, if all that shows on eBay is $20.50, but that bidder has an actual high bid that is higher, the proxy system will adjust your bid amount to achieve the high bid or until your max bid is reached.
- Craig