On the other side of the coin, I have recently bought a lot of BIN items.

I put them all in my basket, then pay immediately I have finished shopping

I see buy it now as also meaning pay for it now.

On 2 occasions, the sellers have sent refunds for overpaid postage.

I have noticed though, that it is sometimes cheaper to send items separately, due to RM`s pricing policy

ie. one item in an envelope or jiffy bag can be sent at large letter rate, but put 2 items together and it has to be sent at small parcel rate, which is more expensive than 2 large letters.