Really? In what other ordinary retail business do you need to get a customer zip code before you can quote them an accurate price on a standard offering?
Practically all of them. I don't see the final price from pretty much any online store until I put in my ZIP as otherwise it doesn't know shipping or taxes.
But at least I can see the final price before I choose to commit to buying the item. Many ISPs won't tell you what the bill is going to be until they've already wired you up for service.
Don't they need to know your address anyways to check if they can provide service at all?
Zip code isn't nearly enough information to work out telecom taxes/fees, as they often cross political boundaries with potentially different rates and charges. You need a full street address.
Same for sales tax, if you are selling in a way where the applicable sales tax is where the customer lives rather than where the seller is.