Learned this last year when posting several parcels home for xmas, from a very helpful woman at my local post office. Thought that with the current restrictions and xmas, more people might be posting gifts rather than traveling so it might come in handy!
If you're postinf several parcels to the same adress (max 15 i think) you can ask for it to be posted as a consignment, which treats it as one parcel rather than the multiple, so saves lots of money!
Not sure how common knowledge this is, but i was amazed when i was told so want to pass on the possible savings :)