Getting a good designer, that wont charge you an arm and a leg, can be difficult. I use to work for a (mostly small business) printing company. Sometimes people would have designers already, and others would ask us to design for them. There were also a select few who did everything themselves and often did so in Word. It is somewhat difficult (but by no means impossible) to print from Word. I personally find Word overly difficult to work with, and this is from a person who use to do this for a living.
I wouldn't recommend working in Word. It is very difficult to adjust items in Word, so you'd want it perfect before you gave it to a printer (or you might get charged for some design time). Indesign is the easiest to use, but is very expensive. Corel is a bit cheaper, but is still several hundred dollars. For all of these programs (including Word) you would have to learn how to use them.
I would recommend using the designer at the printing place where you are getting your postcards/bookmarks done. You should look for a printer that does mainly digital work. Minutmanpress is a franchise that I'd recommend (not just because I use to work there, but also because I like their practices). You will get a better discount if you do everything in one place. The better idea you have about how you want something to look, the less time it will take to design. The lower the design time ---> the cheaper the design. Bookmarks and postcards shouldn't take too long to design. Usually under an hour, so you'd have to pay around maybe $55 for the design. If you get more than one design done at once, and want similar designs for everything, then it doesn't take much more time to design the other items.
The more you figure out before you go in, the better. Many designers will let you stand there and help design your layout, but just be aware that this could lead to more time being added to your design. This isn't always a same day process, so sometimes the print company will work through email to get your feedback on the design. Your designer/printer should be polite. If you have a problem with your designer just know that the design created belongs to you (unless otherwise stated in your contract). If you do switch companies you can ask for the design. It may take some work to get this design, but most designers will (reluctantly) give you the originals. Do a bit of research before you go to any designer/printer. It will save you in the long run.
Ack, sorry for the massive post!