PosyP - yes I have often heard you should roll or fold quilts with the right side outermost. Personally I usually fold with the wrong side outside. Hadn't heard of folding on the diagonal. As much of my work is paper pieced with no regard to grain, I don't worry about that sort of thing so much. Embellishments are the big thing to be careful with when folding.
Majack - I think this is a UK problem. I have been sending parcels (gifts) over to my sister in the UK for 40 years and she has never had to pay anything on them. I do keep the parcels small because of the cost of postage, and always undervalue the contents. I know from the customs point of view, you need to watch what you put for the contents. Posting to NZ is easy, I think things incur GST (VAT) once they are over NZ$200, though it used to be $400.
If folding, I know it seems counter productive but it is better to fold with the right side outermost. Additionally pad the folds and fold on the diagonal rather than with the grain line. Ps majack, the last parcel I had from overseas had an £11 handling charge!
If you are posting to the UK, the biggest problem is the value you fill in on the customs declaration. In the UK we have to pay an additional 20% of the value plus 20% of the postage plus £8 for handling before we can claim an international parcel, unless you put the value at about $15. For other international swaps I do we have always put value $15, gift and quilt block as description plus card, letter etc. I don’t know if other countries have the same problem.
Personally I always fold my quilts for shipping. You could put tissue paper in the folds, but I usually put bubble wrap. Re. rolling it and putting it inside a cardboard tube, a lot depends on the postage where you live and are sending the parcel to. It is usually much more expensive to send a tube than a flat bag.