Just placed an order with screwfix, which came to £45, and a few
pence.

Realising that free delivery is for orders over £50, I was looking
around the for something about £5, which would essentially be free
(given the otherwise £5 delivery charge).

Then I spotted - vouchers, so I added a £5 voucher, which hey presto
takes me above the free delivery threshold, so I get the £5 to spend
next time.