Buyer demanding a discount after payment.

A buyer bought 6 items from me and a few hours after their purchase they sent several messages stating "Consider giving me a discount for these items" and "I have just spent £145 with yourself".

I'm quite irked by this and I feel like they've got me over a barrel because if I do not comply, they'll likely find problems with the items, open a INAD case or leave bad feedback.

What would you do?

ETA: I offer free shipping on all my listings so I can't offer a discount there.