It's worth calling and trying to beg, but since the council have already incurred the expenses of cleaning it up they might hold you to the fine.
Leaving things outside closed shops is deffo considered fly tipping and the shops themselves ask people not to do it. It attracts crime, vermin, puts staff at risk and they can't really take in the stuff for hygiene reasons. (When you think it through, do you really think the possibly unpaid staff want to come in first thing Monday and have to start manhandling and ripping open black bags in the street covered in god knows what? and whatever neat pile you left things in you can bet it didn't stay like that and was prob a scattered eyesore by Monday)