I've always discounted postage on multiple purchases paid at the same time, asking buyers to wait for an invoice before paying. Even where 2 large letters, £1.10 singly became £2.20 when sent together I took off a small amount for saving an envelope and a paypal (micro-account) charge.

In April 2 large letters will rise from £1.10 singly, to £ 2.60 small parcel, 40p more. 2 small parcels at £2.60 singly will rise to a medium parcel at more than £5.20. (I don't know how much more right not, RM's site is down!)  Anyway, my point is that, with possibly the odd exception for 3/4 large letters going together, most if not all multiples sent together will cost more than the same items sent singly.

I currently have the following note in my listings:-  I always combine postage for multiple purchases paid for at the same time, so if you have bought 2 or more items from me please wait for an invoice before you pay.

I'm tempted to simply remove that note and make no reference to multiples, just send them the best way I can and refund after the event if I do manage to save anything.

Also considering free postage but that's another matter.

What are you planning to do? I'm just floundering at the moment and I'm sure bigger sellers than me will have some ideas.