In our opinion these are the best quality boxes on the market and are available in 3 sizes (dimensions listed are measured on the inside of the box):
These boxes are the exact ones we use in our own business and cannot stress enough how good the quality is, we use 1,000s of these per month and we do not experience any problems with weakness/damage.
They are single walled which makes them lightweight and keeps the cost down at the post office. We specified the material of them with a 186g Kraft outer paper and a high performance inner fluting which makes such a difference over inferior quality boxes that we have used before and you might see for sale elsewhere.
Buy any 2 items in one order receive 5% discount, buy any 5 items in one order receive 10% discount (eBay UK registered accounts only), this promotion excludes Multi-Buy promotions.