Just leave the quantity blank and enter the total cost then.
That depends on the work flow you want.
Bank transactions can be imported and bank rules can be created for regular bills, allocation costs to accounts you have set up. The left over non - standard bills can be then manually processed.
If you want Accrual based records you need to also enter invoices. Regular invoices can be efficiently entered by setting up Recurrent invoices. Bank rules can then be set up to allocate payments to the appropriate supplier or customer.
Manager can not currently allocate payments / receipts to a supplier or customer without an invoice, credit note or debit note however this enhancement has been proposed. See Improvement to payment and receipt forms