Sending out 600 postcards. WHERE DO I BUY FROM?

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Hello, 

I just got done sending out about 600 yellow letters, with about a 1% response rate, yet failed to close on any. I spent about 70 cents/letter for hand written letters done by someone else. 

I am still hopeful for my 6 month campaign. 

I want to try sending small 4x6 postcards now. I obviously want to send out personalized postcards with variable data on each (different names and addresses). 

I tried vistaprint, but it doesn't seem to have variable data. It just allows you to put YOUR OWN personalized information. If I'm wrong, can someone point me in the right direction?

For 600 small postcards, click2mail offers it for 53 cents. 

Yellowletters offers it for 65 cents. 

I've heard of people getting it out of the door with everything (print and stamp) for under 40 cents. 

As you can imagine, if I am able to get my yellow letters out for 70 cents, I don't have an incentive to try cheaper postcards, when they are not much cheaper. 

Does anyone have any suggestions or advice on where to buy cheaper postcards? I want to ask local stores, but I feel like they don't do personalized postcards for mail merge. 

Thank you in advance!