It's not so much the postage, a flat rate box isn't bad, but to get the best out of it you need to fill it, that's when the costs start to stack up..
See the link below to help calculate taxes..
New Import Duty & Taxes Calculation | DutyCalculator
Gives you an idea of what it's going to cost..
Which category do you use, I can't see anything that is relevant apart from DIY, Tools & Crafts but then the sub category lets it down.
Peter