The book On Writing Well (a great book,) suggests making things like these plural. In the book, he talks about how to avoid the gender problem when talking about men/women. (By calling them people.) For example, when talking about a specific user, instead of saying "when he clicks on the button..." you'd say "when they click on the button...", or the passive "when the button is clicked...".
Here's how this would work in your case:
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This will work even if you're only ever sending things to a single person. People don't tend find it strange, and it avoids your a(n) problem.
This technique makes the labels a little shorter and keeps them easy to scan. In certain contexts, @matt's answer would also work. It depends on what you're going for.