I sell mainly bulk low end pokemon cards and I cant figure out how to get the combined postage option to work correctly.

I have over 1300 cards for sale and people ask me all the time to combine postage  - and I have the right box ticked and offer £2.85 tracked royal mail but it defaults for each individual sale to the 91p royal 2nd class mail which is the standard i send for - so no matter if people buy 5 or 20 it charges them 91p for each item and for the life of me i cant figure out where im going wrong. Any advice/screenshots of what I need to do would be massively appreciated.

Thanks

Mike