I did something similar at Currys a few years ago when buying a washing machine. It had been hoiked out of the warehouse and was sitting on the showroom floor while the salesman sorted out the paperwork for the till. He then went on and on about having an extended warranty, which I was having to repeatedly refuse. After about the sixth attempt, I said "If you mention extended warranty one more time, I'm walking out the door". He responded with "But the extended warranty is excellent value and...". I interrupted with "I know it's a s*** job but I did warn you, good-bye" and walked out. Went to Hughes, better service all round.
In a similar vein, my dad was buying a great heavy trolley load of slabs at a large DIY store.
He'd struggled to get the whole lot through the store and joined the queue to pay. The lady at the till eyeballed him a couple times but said nothing. He got to the till, she said...
'Sorry sir, you have to pay for those over the heavy side, where you've just come from'
'Wha ?...why can't I pay here ?'
'Sorry sir, (now slightly shirtily)...you've GOT to pay for them over there'
Dad, normally the mildest and best mannered of people said...
'No dear, I haven't GOT to do anything'...and walked out, leaving 1/2 tonne of slabs at the checkout, till woman agog.
Hehehe..very satisfying