the paypal how to doc should tell you how to go about placing the image its an acrobat doc. goto paypla and lok at the merchant features.
I have used this for merchant ebay selling it's ok.
but if you only have one portfolio then perhaps suggest a first desposit first served basis. desposit of x dolalrs locks in portfolio transfer, details to be ironed out...make sure there is a memo number for the paypal invoice