Add any amount option to PayPal payment field

Also, is there a way to insert an additional field to the types of memberships that allows the person to enter a variable amount (like a donation field)?  It would have to be in addition to a membership level field with exact payment amounts.

As a workaround, you may add a separate "Donation" product with $1.00 price and a textbox quantity selector, so that the form user will be able to enter any amount.

If you would like to hide the $1.00 price we can do that with injected CSS.

Also, the Quantity label can be renamed: