AMS doesnt cost anything to set up.... you can go to this link to find out how to register:
Ultimate AMS (Amazon Marketing Services) Guide to Marketing Your Merch - Merch Informer
cost of advertising on AMS really depends on how competitive your keyword is.... if you have a popular keyword like "cat shirt" and your starting bid is 10 cents it's cheaper but your ads are barely gonna get shown. if you bid $1 your ads are gonna get shown more often but it adds up quick if you don't have a daily budget set.
for most lower-medium competition keywords 15-25 cents is a good amount to start bidding....if you notice that you're getting low impressions on a keyword just raise the bid by 10 cents until you notice your impressions start to increase.
Between Redbubble and Zazzle...which one do you think is better for organic traffic?