'Unfortunately, the seller didn't realise that they could change the default Evri courier.'

Yes - this sort of thing is what happens when a completely new system is introduced. There has been so much to have to 'get your head round' with Simple Delivery, it is no wonder that many sellers are still not up to speed with everything. 

A seller can opt to exclude a courier, but that will apply to all their listings - the seller can't choose Evri for low value, or unbreakable items, and select Royal Mail for other items - it is either one courier, or both, with the buyer not able to choose if the seller uses both...  after almost a year, I'm sure many are still finding Simple Delivery confusing & unsatisfactory.