Yes, you are severely underpaid. However, you also suffer from having your fingers in too many pies.
Anyone looking at that list is going to think: Jack-of-all-trades, master of none.
The fact is that in any decently organized business a different person would probably cover each of those responsibilities. Most people will think that the only way a single person can handle all of it is if at least some of those tasks are being blown out of proportion.
In your resume you have to take care to bring out the fact that you were trusted to perform all those tasks while not making it sound unrealistic that you did so.
My Workplace (20XX - Present)
My Job Title
- Hired as Payroll Administrator, and successfully performed this function for the duration of my employment (a sub-list about this part of your responsibilities, etc.)
- Assigned additional responsibilities as Office Manager (indent sub-list of responsibilities)
- Trusted as representative in a court of law alongside our attorney
- Occasionally tackled HR responsibilities by performing he following:(indent sub-list of responsibilities)
Obviously this is a very crude version of what your resume might include.
However, before you send your resume out I would seriously think about the sort of job you want to pursue in the future and include more/less information about the aspects of your job that are relevant to getting the position you're applying for.
If you simply list everything you either risk coming across as over-qualified, or projecting the image that you didn't actually do much in the way of accounting at that company you're currently with (since you had so many other things on your plate).