For example, suppose I'm mailing someone a concert ticket if they are interested in going, and I'm sure they are interested, can I say
If you're interested, which I'm sure you are, I can mail the ticket to you so that you don't have to pick it up yourself.
Specifically, should it be 'which I'm sure you are' or 'as I'm sure you are', and do I need the ',' before it?
Since "which I'm sure you are" is a parenthetical comment, which can be omitted without changing the overall meaning, it should be set off by commas, dashes, or parentheses.
In comments like this, there's no significant difference between "which" and "as".
Grammatically, there is no error; but from the point of view of a detail of logical thinking, perhaps viewed as a matter of mere formalism, but real enough to be noticed, it is not very coherent. One infers readily from the opening clause that the writer makes no assumption about whether the person adressed is interested or not, the writer does not know that since he mentions it as a condition. The next clause is now the assertion that he/she is certain that the answer to this question has to be positive. In consequence, the statement should make allowance for the possibility of the condition not being fufilled; this can be done by introducing the idea of a small incertitude; another solution consist in eliminating the contradiction of what could be a perfect assurance of the condition being fulfilled.
If you're interested, which I can hardly doubt you are, I can mail the ticket to you so that you don't have to pick it up yourself. (very mild doubt)
As you have to be interested, I propose to mail the ticket to you so that you don't have to pick it up yourself. (no doubt)
In the first case the commas are necessary because that clause is a comment on the content of the preceding.
