Hi,
When a HikaShop plugin sends a payment request to a payment gateway, it usually provides the id of the order for which that payment is meant so that when the payment gateway contacts it back, it gives back that id and HikaShop knows which order needs to be confirmed.
Some payment gateways have a security so that it is not possible to pay several times for the same order id.
That might be something possible based on how the ecommerce platform and/or the payment plugin is made.
So then, suppose that you use your merchant account on an ecommerce website... The id of the orders go 1, 2, 3, etc until you decide to switch to HikaShop. With HikaShop, the id of the orders also starts at 1 and then goes 2, 3, etc.
However, because of that security, the payment gateway refuses the payments from HikaShop.
That would also happen if you reset the increment to 0 for the order id after deleting the orders even if you don't change your ecommerce solution.
That however doesn't mean that there is a problem in HikaShop, or that we can do something in the code of HikaShop to avoid that.
Altering the order id to start at 10000 it just a way to bypass the security feature of your payment gateway.
And actually, usually, payment gateways, like PayPal, have an option to turn off that security check.