For my view, there are two type custom make stamps,
1. You have your own design.
2. You use free design.
The material is metal, acrylic, resin and silicone.
Machine only can read machine language, so it is better give AI file or AutoCAD file. If you send image file to stamp maker, they will charge you extra fee.
Normally, people will choose
a. Fonts ---
Some fonts are not available.
If you want to buy stamps, you must take a look at your favor fonts.
Letters "A" " M", are key issue.
Poor quality stamps will see a small triangle, you won't see A or M.
Some soap stamp makers don't make soap, so they won't care what will be.
Some stamp makers are artist, they ignore stamp rule and fun.
Some stamp makers simple resize bigger stamp to avoid above problem. Bigger stamp is more expensive.
b. Design
Most stamps are 35 mm ~ 70 mm length ( 1.37" ~ 2.75")
So, your soap bar size is ?
Stamp size is small, the key issue is
"When soap bars are on table, how far shall see design pattern?"
Too complicated design and thin lines won't easy to see.