Seller fulfilled Prime “buy shipping” question (UK)

Good morning, I’m currently looking into seller fulfilled Prime and a little confused on buy shipping. Do you still have to buy shipping if you already use the courier and the service they use anyway? For example we use DPD, already use an overnight service.

Some documentation says it’s optional to use buy shipping, some is saying you must use 90 odd % of amazons buy shipping for Prime orders?

Is anyone able to confirm? Seems odd that we’d be loading a DPD trailer, to then send a separate van of SFP parcels to exactly the same depot?