In my store I selected the USPS calculator. The problem is it won't give my customers flat rate boxes as an option. Some of them paid $17 for shipping when they could easily pay $12.80 for medium flat rate.
Unfortunately, being able to calculate to use flat rate boxes, if applicable, is not an option in GoCentral. That's because there is no way for USPS to know which of your products would fit in that flat rate box based on weight. Weight alone is not an indicator -- physical size of the item(s) comes into play. And only you know what will fit...
This is where shipping formulas that allow all those type of variations are necessary. Especially if all your products are different dimensions and weights.
If all your products are relatively the same size and weight, what you could consider is setting up your shipping to be more of a flat rate system. For example, you can set up First Class to be a flat rate. Then setup a free Priority say for orders over $50 -- which is pretty common online. For folks who want Priority under $50 you give them the price for what that will be.
If all your products are different sizes and weights, you'll probably want to consider a platform with more robust shipping configuration options such as WordPress + WooCommerce.