Are you going to save the quantity?  Are you going to save the price (since
it may change over time) so that you will always know the total for the
particular invoice?

I would recommend you take a look at the Northwind sample database that
comes with Access.  It includes a form for invoices.  Creating and storing
an invoice is pretty complex.  You typically store the invoice record in one
table, then store the related invoice detail lines in a separate table.  You
also have the issue of using the current price for an item, being able to
give discounts, etc.  And of course there is tax, payment methods, normal
addresses vs. shipping addresses, etc.

All of those issues are handled in the Northwind sample.

Once you take a look at it, post back with specific questions and we can
help you out.

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If you don't want to do that, the simply answer to your question is that you
would create a new textbox on your form and put something like....

=[myQuantityFieldName] * [myPriceFieldName]