You could use the Invoice feature as there is a field for Terms and Conditions.
But, be aware of a few things:
Regardless of the policies you may have, PayPal's User Agreement overrides yours. Granted when you dealing with honest people, this really is not a problem however, when dealing with not so honest people and there's a few out there, you're going to take a loss.
Case in point: Lets say you have a 30 return policy. In the User Agreement, PayPal allows 45 days. The customer has the item for 39 days, contacts you requesting a refund, you explain to the customer that your policy is 30 days.
Customer gets upset, opens a dispute requesting a refund on the 40th day, PayPal does their thing and in most cases, will refund the money to the customer. PayPal does require that the customer is to return the item first before receiving the refund but, they really don't have solid method to enforce that. Customer enters a fake tracking number, PayPal does the refund whether you like it or not. In the mean time, no one actually follows up to verify that you received your item. Customer now has both the item and their money back.
Last, do take the time to go thru the User Agreement. For example, services, intangible items, digital goods and vehicles are not covered for Seller Protection. You assume all risk.
https://www.paypal.com/webapps/mpp/ua/useragreement-full
