Does this combination kit (Amoxicillin 750mg, Omeprazole 20mg and Tinidazole 500mg) only come in these strengths, or are lower or higher doses also available?

For the exact strength options currently available, the product page or customer support can confirm what's in stock. This particular combination is typically used for a set course to treat H. pylori infection, and the strengths are usually fixed as a package rather than adjustable individually. Please don't change the dose or duration on your own - stick to what your doctor prescribed, since incomplete courses can make the infection harder to treat later.