I think you guys are overthinking this form or I am totally misunderstanding what you want.
When I read the first post here you are basically saying you want to be able to tell people to either choose a set amount or have the ability to choose "other" on the radio button. If they choose other you then have a second box show up where they can then insert the amount.
If what I said is correct, then my first post outlines how you would do that. It's all done with the conditionals and you would put a conditional on the input box that you create for them to enter their own amount. Have the conditional show up:
IF
Other "is" chosen on the radio button with the set amounts
AND
IF
10.00 "is not" chosen
AND
IF
20.00 "is not" chosen
And so on down the line for each of your set amounts. Doing each of the set amounts will make the input box not show up under any other circumstances than that they have chosen "Other".
It is that simple for most all conditionals, but I have not tried this with a payment form so test it well to be sure it works as intended.
If I am still misunderstanding your original post then ignore the above and I will stop watching this thread