Having read the guide by rebelmoore I couldn't agree more. I not so long back took my parcels already neat and stamped to just be signed off by my usually good postmaster only to be turned away because I was losing them business by using paypal postage. I then went to a 2nd post office to have the snotty postmaster tell me my parcel was 10g out and I had to take the parcel back, refund paypal and redo the whole thing. Needless to say my business went elsewhere. I have however found a wonderful post office within driving distance who is happy to take in all my parcels, stamp them and without any problems.
I do have a bit of useful info for those of you who do experience this in the future. All post offices and sub post offices were issued with a letter from Royal Mail stating that as the fees from paypal are through royal mail they are not allowed to turn you away. You are quite within your right to make a complaint to Royal Mail who will then warn the post office concerned.