Bill of materials and non inventory items

We have a vendor that does welding for us and we pay him by unit, i.e 20 welds.

1- I have created weld under Miscellaneous items with allow in PO and BOM

2- When I try to create a PO for the vendor, it does not allow me to enter quantity.

3- In bill of material however, I can enter quantity when I assemble my finished good which has raw material plus weld charge.

What am I doing wrong? Is it correct to have weld charge as a Misc. Item to begin with?

Any help with this is appreciated.

Fred