Good for you! I’m impressed with your drive to get this going and obviously being successful in your efforts thus far. Cost-wise, selling on Walmart is very close to Amazon, except you don’t have the monthly account fee. Like Amazon, Walmart’s seller fees vary by category and for the most part, they’re the same percentages of your sale as Amazon.
You do have to apply to sell on Walmart.com and you must have a track record of online sales, which it sounds like you do. Another caveat of Walmart is the listings can be a little harder to set up than Amazon’s. To upload many items, you need to set up a feed which is a technical job. But if you sell just a few products, you can create them one by one.
The first step is just applying to sell on Walmart.com. This costs you nothing and there’s no risk if you get accepted and decide not to sell there after all. I say go for it!