Actually this situation happens a lot where you have an invoice with no Line items. Method user starts to enter in a record, then decides doesn't want it. I agree it shouldn't be that wsay but it happens.
Once a Customer is entered then Method "saves" the record to generate a Invoice or SO header record.
I agree that a "Save" should update the values whether it syncs or not. If you have one item it will save appropiately, even without sync. Since Totals are maintained in the header recdord - I would expect a "save" action to update those values to 00.00.
If a document has no line items then Subtotal is blank, tax and total have 0.00.
Justin is correct, best if you want to delete all lines. to just delete the document.