Zazzle lets you set a default percentage for items and you can leave every item you create priced at that percent, but I suggest that you don't.
In my opinion, many products Zazzle offers are expensive enough without me adding a high markup on top of that. We have to add at least 5%, and we can choose any amount, but keeping the markup reasonable will help with selling.
In my wedding stores I set my invitations at a higher markup than the coordinating RSVP cards, postage, stickers and other items that match because I feel that if the customer buys the invitations they will be more likely to choose the matching items if they are reasonably priced.
Your markup will depend on how much work you put into the design, how original it is, and how much in demand it will be. At the present time, our price changes will go into effect around the 20th of each month, no matter when you make the changes, so be careful when setting prices. Also, once a product is made (or a bunch if you use Quick Create) you have 24 hours to set your prices that will have to stick of a month.