Couple of questions, first is why you fitted an extra level of security to the fuel filler?, is fuel theft a big problem in your area?. Second is why don't you bring it up with the garage?

It may simply be a lack of communication. If it is a very small garage where the mechanic who did the work also did the invoice, that would seem very suspect (unless there is a means of adding the additive somewhere along the fuel lines, which I doubt but I'm not a mechanic!). If it is a bigger garage and fuel additive is something which is added as a matter of course, whoever does the invoices would have no reason to expect they wouldn't actually be able to put the additive in. And if the place is busy, the mechanic may simply have forgot to mention it.