I'll try and explain this as best I can but it might not make much sense with out seeing if.
When I mark my orders as complete they all move over to the completed section. The orders will move over to the completed section like this (made up names)
Order 1 (Tom H)
Order 2 (Sarah B)
Order 3 (Tilly F)
Order 4 (Dylan O)
But then when I go back into my completed orders they are in a totally different order and when I buy Esty shipping labels it prints them out in a different order again.
Order 2 (Sarah B)
Order 1 (Tom H)
Order 4 (Dylan O)
Order 3 (Tilly F)
Its only a small issue but is causing me a headache when trying to go through orders and I feel I'm constantly having to sort labels and rearrange the orders.
Look at the columns on the right of the completed orders page where you can select how they are sorted. Date completed, newest, oldest etc. Make sure you select the same as you have selected on your open orders page and they should then match up.