Storage Notes: Note that this ketchup should last a week, but perhaps up to 3 weeks in the fridge, but I personally wouldn't push it that far. Two weeks is more likely how long it will keep for.
For long storage of up to 6 months, store in the freezer in an air-tight container.
Sweetener: Any sweetener (healthier or low-carb or otherwise) can be substituted for the low-carb sweetener and/or stevia. Read How to Substitute Sweeteners for tips/ratios. Vegetable glycerine is another great option. If you'd prefer to use only one sweetener for both, that's fine too. Yacon syrup or a dash of stevia extract can work too--or even monk extract. Use 1/2 - 3/4 the amount of honey or maple syrup.
Measuring Tips: The measurements for some of the ingredients are small (like 1/64 teaspoon / 1 drop, 3/16 teaspoon). You can either eyeball it (the recipe will likely be just fine as long as you don't really overdo it) or you can buy these handy measuring spoons. I use them for measuring stevia all the time.