Well, you are sort of limited in presentation wise by the zazzle format and the "basic" cafepress shop format. That's not to say you can't make good sales with either, you just have to work around it
The first step would be to work on the "presale" on your billtoonshere.com website.
Instead of linking with simple buttons to the main "shop at zazzle" and "Shop at cafepress" pages which makes the customer wander around to find what they might want, you should create a separate page (or pages) on your site with your cartoons actually on products (like a shop or store section on your own website). Showcase a handful of popular product images like t-shirts, greeting cards, etc, and then make sure to link those product images (and text under it) to the SPECIFIC page on Zazzle or CafePress that a customer can click "add to cart" and buy that product. That will save the customer the hassle of searching around at the zazzle/cafepress site and possibly getting confused by it all. Shorten the number of clicks to the sale on your end and you'll be helping them out and your sales out
The next step I would take would be more involved. This one might take some time to implement, but I think it would pay off for you in the end.
On each of your cartoon showcase pages, you should add a link to where a customer can buy that specific cartoon at zazzle/cafepress. Like this page of your site should have "Buy T-Shirts, Cards and Other Gifts with this Cartoon on it here" and it would link to the SPECIFIC page on cafepress/zazzle with that cartoon for sale.
http://www.billtoonshere.com/Cartoons/Birthday/birthday.html
On each specific cartoon page should be the same kind of link:
http://www.billtoonshere.com/Cartoons/Birthday/BT0001.html
If you wanted to be more detailed, you could add some little icons under each cartoon with some leader text that said "Buy this Cartoon Now On:" and then had a little t-shirt icon, greeting card icon, mousepad icon, poster icon, etc that a customer could click and go directly to the SPECIFIC page with that design on it. The key here is sending the customer DEEP into your cafepress/zazzle store instead of the store homepage. As deep as possible (for example the words "buy this design" could be linked to the page at cafepress/zazzle that showcased the design and the icons could be linked to the specific product pages with the add to cart button)
To make it even easier for your customers, I would suggest consolidating your store to one place. I don't think general customers know what a "zazzle" or a "cafepress" is, so you might be confusing customers who don't know which link to click to buy what they want. And if they want something that's in both places, they might get further confused.
I would suggest either going with a zazzle store that offers unlimited products and take the time to setup sections, or a premium cafepress store that will allow you unlimited products with the same design, more customization options and an easy way to create dozens of products with a click.
For example, with one cafepress premium store, you would then be able to link to specific design pages and specific product pages in one source. CafePress lets you easily setup sections and add your design to mutiple products at once (and markup prices/descriptions all at once).
Then your wording would change from "buy at zazzle" or "buy at cafepress" to more specific words that the general population would understand "buy t-shirts with my cartoons" here (and link to a section at cafepress that has all your t-shirts listed...cafepress has special links for this).
Sorry for the long post
I think your site and designs have a lot of potential. Being a published cartoonist gives you a bit of a following, traffic, and people naturually interested what you have to offer.
Any other questions, feel free to ask.
