I see nothing wrong with handling domain registration/updates or SSL registration/updates if the client want you to handle that.
With that said, the way we approach this is that we pass along the actual cost to the client for the domain/SSL + any billable time. I.e. we also charge them for the time we spent on doing this update (billed at half hour intervals).
With the hourly fee we charge, the additional fee we would need to append for these services would make it look more expensive than billing it this way (hourly). So make certain you compare the time you require to do this vs your hourly rate, when deciding on what additional fee you charge for this service.
Remember if you can spend the same amount of time doing billable work at a higher rate, you should do so! So unless you charge the same for doing this service, it would not be a good idea to offer it.
On a side note, the type of clients you have also decide how much you can charge. With the right type of clients, you can have a markup on over 10 times the price or more compared to if they purchased it themselves, which is why you want to bill according to your hourly price compared to with a set fee. Since if you tell the client it will cost $10 for the domain, and $100 for you doing it, it sounds expensive since they compare the price of the item vs your fee. While if you tell them it cost $10 for the domain + hourly on the time it takes to set it up, they will have no problem paying $110 even when the record is noted separated on the invoice. (This is of course assuming the client is already familiar with the hourly rate you charge and have no problems with paying your invoices.)