feat: invoice payment functionality

[DioChuks]

Description

Wasnt accounted for in the earlier push, so i've added comments to close this.
Amendment also done already.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Code refactoring
• Test improvements
• Other (please describe):

Related Issues

Closes #75
Closes #134

Changes Made

• added no new changes
• only comments

Testing

• I have added tests that prove my fix is effective or that my feature works
• All existing tests pass locally
• I have tested this manually

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published

Screenshots (if applicable)

Additional Notes

[0xDeon]

Lgtm