What are the some available defenses against product liability claims?

I am doing freelancing for two different companies and I have several questions. I deliver for them digital files, such as research, PowerPoints or excel files. Do I need to add sales tax to the invoices? When and how do I file taxes? I have never worked before in California and just started working in October last year.

What exactly is accounts receivable financing and factoring and how can it help a business?

How many people actually go in for a Health Insurance? It is observed that most people think it is a waste of money so avoid it. What do you think?

If I'm participating in my employer's 401K plan, may I also set up a SEP-IRA for my self-employment income?

What are the advantages of factoring?
Factoring allows your company to get instant cash, in less than 24 hours. Factoring can be defined as a process when a third party firm, known as a factor, purchases the accounts receivable of a business and gives it cash advance based on those bills (that are unpaid). It has a number of benefits like:
1. Does not require collateral
2. Good use for growth
3. Qualify for more and more funding
Do you agree with this? Are there any more benefits?

What is the difference between the gross and net interest rate?

Is a credit card statement considered a viable replacement of a receipt for income tax documentation of expenses or are credit card slips still required?