Nestled Selection on Mobile

I'm designing an e-commerce product page where the user can select to pay today or pay monthly. The option to pay monthly has several sub-options (18 months, 24 months, etc).

What is the best way to present nestled selection options? Currently, we're using radio buttons for the choice between today vs monthly. Should the monthly sub-options also be radio buttons or should they be checkboxes (only one option can be selected at a time)? I prefer checks for the visual distinction but checks are usually reserved for multiple selections.

Option 1 Option 2