THank you for the reply but there are a few issues.

1) I already use up 3 custom boxes (that is the limit I believe). I cannot change this as I need those 3 boxes

2) Even if this is changed, are your instructions to add SKU as a custom field and then for every entry on an invoice I would have to manually enter each sku? We have hundreds of products - this is not possible to remember each SKU - it should pull up automatically.