Stop with the "agile" Fail Fast, Fail Often software methodology... You are driving customers away.
Can you just...
Get everyone back onto DMO etc.
Reset the s/w project & document what you are going to do & how customer systems will interact with it (for ALL features).
Code, test and stress test the new system internally at length.
Then release it to those that want to use it, with a carrot of reduced prices, reduced time to produce postage, better system security & other advantages and WITHOUT the stick of canning the DMO system. (no more "you must move NOW!")
Without it, you will see a bleed of customers trying and moving to other shipment providers...
Farhad Abbasi commented
We have been using Click & Drop for 1 years now it isn't new and it is quicker for me because we can have our orders from Ebay direct into Click & Drop.