Should I prompt user for total or subtotal?

I am updating a system for my company to automate business expense entries into accounting software. Currently, users create expense records in a web form that requires the total amount for the expense. In order to completely automate the process for adding the record to the accounting software, shipping and sales tax amounts are needed. Since we are not using any sort of software or library that scrapes the data from a receipt or invoice, these values will need to come from the user. The users for the system are pretty familiar with it, as 90% (or more) of the company uses it for expense entries. The company is small, so user education is simplified.

View of the current interface:

The updated interface would likely have the PO and Invoice fields moved down into their own line/row, and the new fields would be added after Amount, pushing the --paid with-- selection to the right.

Should I continue to prompt the user for the total amount of the expense or instead prompt for the subtotal?