I think the easiest way to put it is this:
I have the site set up so that (for UK postage):
1) If you purchase 1-5 posters that can be rolled, then the charge is say £5
2) If you were to purchase 4 posters that could be rolled and 1 that was flat then the postage above would be increased to £9
3) If you purchase 6 or more posters that can be rolled then the charge is £8
4) If you purchased 6 posters that could be rolled and 2 that were flat then the postage above would be increased to £12
However, if you only purchased 1 poster that was flat and no other poster then the postage would be the same as example 2. There are only the 4 basic rates for UK and another 4 using the same ratios for International.
Basically, it is due to the postage rates and sizes of the posters that cause the change in the rate that has to be offered when a flat poster is purchased in any order. The mere presence of a flat poster in the order should change examples 1 and 3 into 2 and 4
For instance, someone just ordered 6 posters to be sent to America - 1 of which was flat. The flat rate to send the rolled posters is £8 but when I took the parcel to the post office with the posters wrapped flat the postage went up to £30! So I need to make sure that in future if a flat poster is added to the order then a higher postage rate takes over.
I hope I have explained that clearly. If not then just let me know
