TPM;29240 wrote:True, but my standard mugs always seem to cost £2.48 (I think it is) upto around 500g (I think) - I dont think I have sent anything less then 250g

Yes, but you need to think outside the box (pun not intended!).
If you send two mugs together then they weigh under 750g if sent in polystyrene boxes, but over 750g in smashproof boxes. Postage costs more.
Send three mugs together and they can go for under 1Kg via 2nd class post if in polystyrene boxes - but in cardboard boxes, they're over 1Kg and so can't go by 2nd class post anymore. The difference between 3 poly-packed mugs 2nd class post 1Kg is £1.75 or more. That's one heck of a chunk.
If you send a six-pack of mugs, then they can go under 2Kg in polystyrene but are over 2Kg in cardboard. That's another £1.77 difference (or a staggering £13.65 if your customer wants Special Delivery).
Smashproof boxes are great in certain circumstances, but when weight is a consideration they're terrible. Best to have both options to hand.