If you have flat rate or threshold based shipping – the shipping part is easy. Taxes become tricky when you have multiple locations across various states (assuming we are talking about a US retailer). The answer for that is to do pre-checkout shipping/tax estimates – this requires a tax rate lookup table (companies like eSalesTax offer this).

You ask a user to enter the address zip code on the cart page (this is one field exclusive of all other personal info, so resistance is minimal). Query the tax and shipping tables – return available shipping options and rates (standard, overnight, etc.) – default to lowest cost rate – display total (including taxes).

All of this ideally uses Ajax to maintain the users page position and for speed. If a user switches shipping types, simply re-calculate total based on such – easy.

Shipping requires the web site to know whether you’re shipping standard, ground, next day, 2-day, and so on.

Sometimes shipping is taxed, and sometimes it’s not.

What do you think web sites should do – give an estimated total before you hand over your shipping address? I’m thinking that might be useful with a caveat of “this is estimated – we won’t know until we get your ZIP code.”