With the current set-up, on the 3rd panel (Item Details) it should have packaging options, which you'll probably need to edit.

If you click on Edit, the panel will expand and there's a field labeled Product Promo code. Enter the satchel code there (eg for the 500gm satchel, the code is 39712, which you'll find printed on the back). Click "apply" and the other fields should be pre-filled with the satchel specs. Then click 'Find Available Services'. 

If you primarily use one packaging type/size, you can select to save the item details so that the info for it is set as the default and you'll only have to edit the item details if you use a different packaging type or size.