I am working on an bitcoin payment system that generates invoices, because the invoice is time sensitive we like to control the fees our users pay as much as possible.

Is it possible to create a BIP70 payment request that specifies the fee the payee is expected to pay (either per output to be summed up, or in total for the transaction)? We can do our own validation, this question is just about the creation of the payment request (if the bip70 spec allows for the specifying of fees).