I'm running v.1.3.8

I'm setting up a sponsorship type item. Currently I have the item setup as a radio button with amounts of $50,$100, $200, $500. I'd also like to have a radio button option that the customer can select and enter in the amount they'd like to sponsor. Is this possible within the same item?

Basically look like this

A. $50
B. $100
C. $200
D. $500
E. <Empty box to enter amount>