I have heard that post cards need to be simple, like promote only one thing so that it does not distract the customer and keep their attention on the 1 service you want to promote.
I have heard from marketing professionals that you can make cards look pretty and if they don't have a good marketing message, there is no reason for them to call.
You need to have a good marketing message or it is just another postcard that will get thrown away like all the other worthless junkmail that people get every day.
That is the biggest problem I have found, how to get a good marketing message as I and most people I know are not professional marketers and our opinion of what we think will work is not what will really work out in the field unfortunately.
I have had postcards made in the past by reputable companies and they looked great but not a lot of calls from them and less jobs from those calls but now I realize it is because even though it had nice pictures and was put together nicely, there was no good marketing message on the card, in other words, no reason for them to call.
You can buy blank or generic cards or cards that supposedly work for some people and find out after spending hundreds or thousands that they were not professionally made by marketers and were just a huge waste of time and money since nobody called when you sent them out.
One way to see if something might work is hand one of them over to strangers (no friends or family because they will tell you what you want to hear, not the truth) and ask them if they would call for this product or service if they got the card in the mail and see what they say. If they hesitate, that is a no. Any answer than yes is a no and you have your answer.
I think that if you had some done by a professional marketer (not a wannabe marketer or self-proclaimed marketer or anyone else that says they can do the marketing for you), get some of them and ask people and see what kind of results you get, you might have to change the message around and do some testing until you get a good message, then it will be golden and should get you a lot of calls and work.