But what if you need to get a little fancier and include a drop-down menu for different sizes or colors (or anything else)? Then you should build your Add-to-Cart buttons using PayPal's simple button generator and add the buttons to your site via our Custom HTML Element.
PayPal walks through the process of creating payment buttons here:
Click the "Create Your Button Link" on the above page to do just that. Their create button tool gives you significantly more options than our built-in e-commerce elements.
Note that you do not need (and should not) use our own provided e-commerce elements in conjunction with PayPal generated buttons. This will cause confusion and make it so nothing really works properly. Which is likely the opposite of what you want.
You should instead create a description of each item using our Paragraph w/ Picture element and then line each description up with the appropriate Custom HTML Button.