For such a small item and the small load it will carry, even 50 percent is substantial. Keep in mind that one hundred percent infill is not necessarily stronger. If you need to know why, consider a 'net search for "why not use 100% infill" for more detailed information. The primary foundation for not using 100% infill is that the stress is better distributed over the structure of a non-100% part, while the completely filled part has more intra-layer stress failure. Another link suggests that there's a possibility of increased warping with full infill.
Because the load is small, it matters very little if you select PLA over ABS. PLA is more brittle compared to ABS and will crack or fracture or break under loads that might otherwise cause the same part in ABS to bend.
If you need yet more strength, select PETG or nylon, although I suspect either one would be more expensive from a service.
If you select ABS, you can use acetone smoothing later to make a shiny surface, but that's cosmetic, not structural.