If you don't have a written contract then it depends on the terms of your oral agreement. Did they agree to pay all hours of your time, including travel, or was it just to pay for time worked? Without a written contract you are going to face some hurdles, but it all depends on the agreement and then what evidence you have to support that agreement.
As to not being paid on time or the correct amount, that brings up the same issues discussed above. If they are not paying as agreed you can consider suing them.
I strongly recommend that you only do work by written contract or you face problems like this. The cost for having that done by an attorney is a fraction of the time and hassle you will have if you just do things by handshake.
This answer is for informational purposes only and is not legal advice regarding your question and does not establish an attorney-client relationship.
You list "employee rights" as a topic, but you have to understand that you are not an employee.
If you are properly classified as an independent contractor, it basically means that you run your own business. This is how you need to think about it. Your business has been engaged to perform services for the company you are working for. Whether they pay for your travel when they hire your business will depend on the agreement you have with them. If you have never clearly discussed these issues, it is past time that you do so. Once you reach an agreement, it would be best to get that agreement in writing (or at least outlined in email) so that you can prove and enforce your contractual agreement in court if it is not honored.
You can reach Colorado Legal Solutions by phone or email. Stephen Harkess is an attorney licensed in the state and federal courts of Colorado. This answer is for general information only and does not create an attorney client relationship between Stephen Harkess or Colorado Legal Solutions and any person. Sometimes free advice is only worth as much as you paid for it. You should schedule a consultation with an attorney to discuss the specifics of your legal issues.
There is no set rule. It totally depends on what you have or had worked out either orally or in writing or through prior common practice.
The information provided in this answer does not create an attorney-client relationship and is not considered to be legal advice.