I usually ask for payment within 3 days but you never know - some family emergency may have cropped up, internet may be down (mine was for 26 hours a couple of weeks ago), illness etc. or they may be waiting for payday to have the money.

The immediate payment option would be your best option in the future & to my knowledge does not cost any extra - I've never used it on a listing!

just another thought - DID you send an invoice? Some buyers even though it is 1 item & postage is listed still wait for an invoice to arrive!