Sugar water mixtures have always worked for me. I've experimented with adding drops of honey, different kinds of sugar, maple syrup, etc. and have had much success. I guess I don't really understand why I should go out of my way to buy sunburst as opposed to mixing something of my own, which is cheaper, just as convenient, and merely a matter of turning on the kettle and finding a jar. Plus, I don't have to wait for shipping.
Novelty items like feeders and formicaria are different—they are harder to construct alone, and it is more efficient to purchase one from someone who knows what they're doing and does it well (hence, the success of byFormica liquid feeders). I guess, maybe do a better job of advertising what makes sunburst special. On Amazon, the only listed distinguishing feature is "In addition to sugar, Sunburst supplies the perfect balance of electrolytes to stimulate appetite and promote natural feeding response", although I am not really sure what this means; perhaps go into a little more detail.
Of all the people who visit Amazon to purchase ant supplies, frankly, many are parents who want to buy supplies for their kids who watch AC and just caught a queen. I don't know the numbers obviously, but while a good chunk do purchase Sunburst, you're missing out on the customers who settle for "I'm not going to pay $20 for some sugar water, I can just use honey. They're ants, for god's sake!".
I know you get a lot of exposure on Amazon, but I think a website or even a blog could go a long way. You could experiment with different sugar:water ratios, different colors, perhaps market to specific species (e.g. "Good for Camponotus", "Works really well with Aphaenogaster"), and give your customers an opportunity to learn more about why buying from you is really that good of an idea.