How to add a user definable payment amount to a form?

The ability to ask the user for a an amount they wish to pay has been added so this can be done on all Payment systems. If you click that you want a user defined amount with the check box shown below it will expand to show additional fields like the regular expression/messages to be shown if the way it is entered is invalid/incomplete - NB whilst you can change the regular expression/messages I'd only do this if there are problems with integration because this regex is what the APIs expect.

There is also the ability to have a user enter a reference and for this reference to have its own regular expression. This will be very useful in ensuring account numbers/license references etc are valid. 

The good thing with this field is that you could have a list of payment types a person could make on a page. Then, based upon the option they select show the relevant ad hoc user defined payment element.