Order entry and Purchase Order using VBA

Hello,
I have a catalog sheet (Order.xlsx) and purchase order (PO.xlsx) that need to link together
automaticly to create a purchase order that can be added, and manually modified by the user.
The catalog sheet has pull-down validation to select the items to build the catalog number.
Then user will click "add to cart", the catlog number will be automaticly added to PO format.
The item will lookup in the range of rows for an empty row below the existing line item.

The customer can go back to reorder another item. See more detail in the attached sheet.

It seems difficult or not sure it is impossible to write in VBA. Please feel free to make better.
It is similar to buy mail order from web. My thought is to avoid typing and automation to avoid
human error. Please note I place two seperate workbooks into one for easier linking since I am
good at all in VBA.

I hope I do not ask too much. Thanks for the help.

Best Regards,