Our new shop has many variations of weights and boxes. it's been a challenge as shopify doesn't have the shipping options we need. Adding shipping into our products pricing would only deter sales. Boxify had the app we needed. Dave at Boxify has gone over and above to explain things to me and help us thru this as there are many details to setting this up correctly. but if you follow what they say-your shipping can be pretty accurate. You can also add a fair shipping & handling fee into the price that blends in to the shipping cost for the client. This app is spot on with USPS pricing-if you set it up the right way. I would definitely recommend them.
Tip-have patience with this app because it can work.

I love Boxify! We ship all over the world and this app helps SO much with quick shipping calculations. It has significantly cut down on the time we spend looking up shipping costs for our customers.

Whenever there is an issue, the support team responds back within minutes and the problem is solved. And, don't get me wrong, the problems are not with the app, but stuff on our end that we didn't input correctly. The Boxify folks take the time to look at all big picture and can spot our errors. Thanks guys!

Boxify has been an incredible addition to our Shopify ecommerce site! Our product line consists of 14 products varying greatly in size, shape, and weight. Before incorporating Boxify, the calculated shipping rates our customers were getting at checkout were wildly inaccurate. Now that we have Boxify up and running both our customer and our company are saving money, AND our 3PL appreciates being told exactly which size box to use so there is no guess-work. The support staff at Boxify is responsive and so super helpful. I highly highly recommend this App!