Different mobile devices/mobile OS load different keyboards for our forms; sometimes, they do not contain a decimal point key.
We have a custom code snippet that will change the type of the custom amount field to a number field and will display the decimal point on most devices:
https://library.wpcode.com/snippet/rodjlwom/
For custom code snippets, we suggest WP Code, one of AM’s other plugins.
It may also be helpful to suggest this article from WP Beginner for inexperienced users
See our documentation for reference: https://docs.wpsimplepay.com/articles/adding-custom-code/