I sell a bit on eBay, and on lists on Soul Source etc... but i also have a full time job, so sometimes I'm not able to get to the Post Office during opening hours...
I try to mail out midweek and Saturdays, but on occaision it is only on Saturdays, so if you paid one Saturday teatime, and it wasn't mailed until the following Sat morning, factor in Sunday and a couple of days on top for the Post Office to deliver it and your up to ten days already...
I wouldn't personally enquire about a record until it was 14 days after payment, and only start a claim after 28 days, as 'shit happens'; maybe I'm a little more forgiving than most, but standard 'delivery' times are within 28 days of payment for businesses, so I don't see why it should be any different for record dealers or 'hobby' sellers.