Macro for adding cell values to end of a list?

I'm working on a small invoice-creation/inventory-status system. Here's how it works:

There's a sheet(we'll call it SheetA) with an empty invoice template that has to be filled out. A button (ButtonA) runs a macro (MacroA) that creates a copy of SheetA (with its values) and then clears out the values on SheetA so it can be filled out again to create a new invoice.

I want to create a button (ButtonB) that runs a macro (MacroB) which takes the values from specified cells in the filled out form (before using ButtonA, which runs MacroA which clear/deletes these values) and places these values in another sheet (SheetB). Values such as quantities being sold and invoice ID number.

This can easily be done by creating a macro that copies and pastes these values from SheetA into specified cells in SheetB; but that will only work for the first invoice created. When a new invoice is created, the values would be placed in the same cells, replacing the values from the last invoice. My problem lies here:

The purpose here is to create a vertical list of quantities sold, next to their corresponding invoice ID number, so these can be added up and subtracted from inventory in existence. Now, what happens when a new invoice is created and I want these values to be placed BELOW the cells from the last invoice created? Is there a way to program MacroB so that every time a new invoice is created, the cell values are added at the end of the list being created in SheetB?