1. A computer implemented method for effecting a transaction between a buyer and a seller comprising: defining a set of fields for the seller's invoice in a computer database; defining an invoice format based on a subset of the set of fields selected by the buyer; based on selection received in a system associated with the buyer, defining a set of rules for accepting information into respective fields of the invoice, wherein one of the rules determines whether a party to whom funds are to be paid is able to input information into the invoice; accepting information from the seller for fields of the invoice based on the rules; notifying the seller if information provided by the seller is not acceptable based on the rules; providing to the buyer the invoice with the accepted information; and effecting electronic payment from the buyer to the seller based on the invoice.