The Shirt Off My Back

I pop into our local 5 à Sec on my way up to the business school in the early afternoon.
'Is that it? 4 shirts?', says the lovely lady who works there.
'Yes..and can you have them ready for Monday afternoon?'
'Fine, but you won't get the special offer...that's for 5 shirts!' A pause...while the Scotsman thinks.
'I can come back with this one in two hours!', I say pinching the front of the shirt I'm wearing.
'With that? Are you sure you'll remember?'
'Don't worry...'
'We'll go mad looking for the missing shirt if you don't and we've issued a ticket for five.'
'I said, ''Don't worry!'' ' And, yes, I did remember and, no, I didn't go straight to the shop on my return and strip off, instead taking a little detour to our house 100 meters away!