For myself I think if I was a customer I'd like having the option especially if, as my plan is, for the discount offered to be exclusive (at least for quite a while). As long as its a one - off I can't see it annoying anyone.
As anyone who's ever bought anything online will know, the bombardment from some online retailers is pretty relentless! I wouldn't want to be mailing several times about it like they do, that WOULD be annoying.