If I'm not too late to add some advice:
The more ‘touches’ you have with a prospect before asking them to do anything, the more likely it is they will respond as you want. The fact that you’re purchasing a mailing list makes me think that the postcard invite will be the first thing they hear from you?
If that is the case I’d suggest (as a minimum) calling people the day before you expect them to receive their invitation. This will be a soft call (not a sales call) just to let them know that they can expect a special invitation in the post tomorrow. Be prepared to give a brief explanation of what the event is and how they will benefit from attending.
Try to make the call a little personal and mention that other local businesses in the area will be coming so there’s a chance to network.
Also, during the call, ask them if it would be ok if you call again in a few days’ time to check they got the invitation if you haven’t received their RSVP back. Again keep it soft and guarantee to them that you will only call once and not badger them each day up until the event.
If your mailer is going out to hundreds of people then you probably won’t be able to do this to every invitee (unless you outsource it) so cherry pick the businesses you really want to be there and focus on them.
As far as what action you should take to get them to respond I would give as many options as possible. Give them an email address to use, put a phone number on there and put a web address on there that takes them to a sign up form on your website (keep the form short – just name and email).
Ideally, no matter which method they use to register you need to get their name and email address - that is gold for your sales funnel going forward.
As the event approaches make sure you give people a call/drop them an email to remind them about the event. Again, if you’re able to talk to them they’re more likely to turn up as you’re building the bond.
Finally, keep in mind a few things:
Direct mail usually only has a 2%-3% success rate; using all the tactics above, plus a slick team, might raise the response rate to about 5% so don’t get too depressed if numbers are small. I used to do seminars for local businesses using mail drops and would send out about 1000 letters for each event if; I got 20 people in the room I was doing well as I knew that at least one was likely to sign up and that would cover my costs and more for the event.
Also keep in mind that even though people say they will come many won’t turn up. Again, roll with it. It can actually be an advantage in some cases as you can call them and say that as they couldn’t turn up how about you visit them at their premises and explain what it was all about?
Hope that all helps.