The order number that comes in from the Prestashop integration module is the internal database ID of the order number in Prestashop's database. Unfortunately, this is printing on the labels as the Order number. This is not the customer-facing order number that customers use to reference their orders. Instead, you should be bringing the Prestashop Order "Reference" over for the purposes of printing on the labels for recipients to see.
Example:
Order ID Reference
1234 BNZAWEOYR
ShippingEasy is printing "Order: 1234" on labels, but it should be printing "Order: BNZAWEOYR" because 1234 means nothing to the recipient.