Dear Sir/Madam,
I understand memory prices fluctuate, but that is completely irrelevant in this case; memory does not go up £40 overnight, it was purely an error on your part. I have also read your terms and conditions and understand what you have said is perfectly within your rights. However, for the sake of good customer service, I would kindly ask you to meet me half way, and I'll pay half the different on top of what I have already paid you. This would work out at £xx.xx/2 = £xx.xx and the total I would pay is £xxx.xx ex VAT.
I hope you can understand my situation and can facilitate my needs on this one occasion.
Regards,
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