In Sweden, I would say "Payable within 30 days" means people would pay pretty much on the 30th day. That's how I would pay my bills as well. The payable day is a matter of contractual agreement and can be 10 days in some instances, or 60 or even 90 days when billing larger corporations (which could kill a small business). This is why some Swedish firms sell their invoices for a discount to some financer, especially in the cases of 60-90 days payable. I have been told that people back in the day would expect a discount (usually 2%) to the invoice if paid upon receipt rather than at the end of the 30 days. In any case I have had no problem stating 10 days payable on invoices, as agreed.