Why don’t brands & retailers offer a choice of delivery carriers?
Why don't brands & retailers provide a choice of multiple delivery carriers at checkout?

It still amazes me that more retailers don't offer a choice of delivery carrier.
When we're in shopping & delivery receiver mode, we all have our most- and least-favourites based on the friendliness of the courier, what time our house usually gets the drop, and how far over the fence they can throw the parcel. And whether they drive an EV or a diesel banger.
So why don't brands & retailers allow us to choose at checkout?
Of course, it's about cost and efficiency. There's a bit of volume pricing, sure... but very few Outbound DC Managers want to juggle three or four carrier pickups each day or night.
Vinted has provided sellers with multi-carrier choice on its platform, for the convenience of both sellers and buyers. In fact, when it tried to force all sellers to offer all carrier options, there was enough uproar to force a back-track.
Has anyone trialled this to look at the impact on efficiencies versus uplift on customer retention? The tech is there. It's really about the importance and priority that a brand or retailer places on its operations, or its customers.