I'm setting up OpenCart for the first time on behalf of a small charity and I'm an enthusiastic amateur, not a pro. We've been using a free version of CubeCart for several years and it no longer meets our needs. Most of our small range of items qualify as large letters, one or two as small parcels; but in some multiple item orders, large letter items have to be sent as small parcels because of increased volume and weight. And, of course, the Royal Mail pricing schemes for large letter/small parcel are quite different. We send worldwide.
Can anyone recommend a not too expensive (preferably free, as we're a charity) method of working out, by measuring weight and volume of a combined order, whether it should go as a large letter or small parcel, and calculate shipping accordingly? I've been looking at category-based shipping extensions but they seem awfully complex for the small amount of business that we do - I don't want to take a sledgehammer to crack a nut.
ETA Update: Johnathan's formula based shipping extension was the solution. Well worth the money and although it was fiddly to set up Johnathan gave good support.