There are a few options to collect this additional pirce of data. To enable custom fields go to the buynow module settings and see the following sections and show/hide custom field options:
- Step 1 Setup - Show textbox 1
- Confirm Page (After Payment) - Show text 1 or show special
To enable custom fields on the product details page go to the product detail module settings and click "Show text 1" and enter a label
-Scott