Hi! This is my first post here, and is just to say that your time also has a value! Returning the SP3 would mean losing from one to several hours, depending on how far away the store is (and the price of gas, in case you drive), plus all the time you need for migrating your data to the new one, which I would also expect to be at the very least one hour, probably several hours.
Theoretically the fairest thing to do would be calling them on the phone and informing them, which is already a favor, so that they can check their inventory and make sure they don't do the same mistake with other people, and then tell them that if they want you will return it, but only if they pay you an amount you deem fair for your time, since the mistake was theirs and you are doing them a favor. They won't of course pay you in cash, but they might give you a voucher, it's up to you to accept that or not. Depending on the time you estimate it will cost you, such amount might easily exceed the difference between the SP3 and the SP5.
Of course most likely they won't agree, but at that point you are entitled to keep it with no remorse of any kind, and I don't believe that they can complain in any way, even if they become aware of the mistake, as you are in no way obliged to fix their mistakes for free.