Your logic is flawed and you are trying to avoid the best feature of Access which is bound forms. You are creating purchase orders and therefore the Products should be limited to those ordered from the specific supplier.
I don't understand why you don't want records to be saved as you add them to the order. The user can always remove an item or fix the quantity if he needs to before the order is transmitted to the supplier later.
1. Naming all your PKs "ID" is not cool nor is it useful. Using appropriate names makes the schema much easier to "see" without having to open the relationship diagram.
2. This form can be designed in two ways and it depends on which will be more efficient for the users. I'll go into the options below
3. The standard price belongs in tblProducts. Otherwise, you need to type it in for each order.
4. The product combo on the subform, needs to be filtered by the SupplierID on the main form so that only products ordered from that supplier show in the list.
5. Why do you have the tab stop set to no for the subform controls?
6. Dirtying the record in the load event is wrong. Only the user should dirty a record. If you want to open a form in either add view or edit view, use a menu that lets you do that. Do NOT put code in the load event to force it.
7. Writing multiple instructions on a single line is also not cool or efficient. It just makes your code hard to read.
Option 1.
Only data validation code is required
Remove the add button and remove the unbound search and qty fields as well as the product listbox.
In the data subform, remove the unbound controls and the FK to the parent form and the ID of the subform. They don't need to be visible.
Option 2.
Code required to control the interface as well as validation code.
Instead of using an add buttton, use the double click event of the list box.
Remove the unbound search and qty boxes.
Add code to the dbl click event to open an input box to get the quantity.
Verify the qty. Then run the append query to add the row to the data table. The append query needs 3 fields = ProductID, qty, and PurchaseID. The PurchaseID is required to connect the row to the order.
I fixed up the some of the problems with the tables like removing the 0 default for IDs and making certain FK's required. I also fixed the entry form to work the way I think it should work which is option1. Feel free to unhide the objects and make the changes I suggesedd if you really want option2.