Use Sales channel order number as Dear order number.
It's not the worst having it as the reference number. But not having the original sales channel number not go through to Shipstation is the worst. I don't want different numbers used for the same order. If a customer calls with an order number and I go straight to Shipstation to see if it has shipped, I wont be able to find it cause Dear uses the invoice number. Dear doesn't use the Sales channel number for fear of duplicate numbers. But that should be my choice and my risk to take on. Other software's allow it. They warn you it could create duplicates but they still allow it. Even if it used the sales channel number but added a prefix to it would be fine.