Yeah it's not just phones, basically it's the Consumer Rights Act 2015 which gives you an option to return an item if you buy it away from the sellers premises within 14 days (most retailers voluntarily extend that to 28 days or beyond). Doesn't apply to in-store purchases since you can inspect the item there and then. Some smaller resellers can be finicky though with requirements as the law only states that the item needs to be in resellable condition. Some will accept it as long as phone looks brand new, others like in this case seem to be difficult about the smallest things. They are being bit funny though, my P30 Pro was wrapped in wrap around plastic type thing and phone is unusable with it on so not sure how you can inspect and try phone with it on. I just wrapped it back up in it when I sent mine off.