Just sold something for £50 but did not use Simple Delivery as I could not see Yodel which is only a short walk.
Delivery is £3.91 for 2kg but only protected for £25 so an extra to protect the extra £25 so £6.40 in all.
If I used Yodel through my own account delivery is still £3.91 but covered up to £50.
I'm not sure if I can work out how much extra to charge in advance but I can probably work it out from recent orders.
Yes I am aware that Royal Mail do you to 10kg protected for £150 for £6.65 and they will collect.