Paul, I do the same with tyres mate. Lay them together, wrap 3 bin liners around them, then over tape with brown parcel tape
Post offices vary dependent on size and how many people work there, IMO. They work on commission so, even though you might have weighed it at home and have your quote, it may not be the same as what they tell you. For example, I boxed a set of bars and made the box around the bars. There was no room to play with. I took them to the nearest post office to me which has around 6 windows. They wanted £25 to post them to the Netherlands, international signed for. Too much. So I then went to the small independent PO in Tolworth with the same boxed item. They have 2 windows. They charged me £14. £11 less than the other thieving arabs
I then tried again with another package to test the theory. I boxed a set of bars, seat post and seat clamp all in the same box. Took it to the main PO and got a quote to post to Canada. £56 international signed for. Took it to the small PO and was quoted £38 for the same thing. Needless to say I do not use the main PO anymore
It's normally around £6.72 per set of tyres sent 1st class recorded. I find that reaches the recipient the next day if posted early enough