Non of the options you mentioned is a fool proof solution of not being charge backed. There is nothing one can do by verifying everything. the best is to raise a invoice from PayPal for a digital goods or service and make the other party pay via that invoice.
Digital goods and services are not covered under Paypal Policy and cannot be charged back.
i know it can't be fool proof as long as there are humans involved. do tell more about this digital goods and services invoice if you have experience with such transactions.