Whatever postage method you use get the post office to put a DO NOT SAFE DROP sticker on all parcels. The recent show on A current affair showing items being left on top of unit letterboxes was an eye opener. That is where a lot of non delivered parcels seem to go missing along with the "I did not receive the item" and that is hard to fight when the item is scanned as it is handed over or picked up from the post office.

Since tracking and DO NOT safe drop stickers have been standard for me "losses" real or imagined have dropped considerably.

I also use a4 flat packs for magazines and books as often you can get it less than 20mm and the corners of the books are protected.

(If you stay friendly with your post office as the flat packs are technically 1cm too long for large letter you need to fudge a bit.)

The A4 flat packs are also useful for larger items as you can fould them out to a box that holds quite a lot. and they cube out less than the standard boxes as you can easily flatten them to the size of the product.

I no longer use self made boxes as the time taken to custom make packaging reduces your efficency considerably.

Try making custom cartons for half an hour and compare that to the cost of buying a ready made product.

If you are selling quantities of items you can save a lot of time and money by using commercial packaging.