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

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.

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?

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

I am a Nonresident Alien and I live in Ensenada Baja California Mexico. I’m about to buy many electronics. Total value would be around $12,000 for my personal use. I am building my own house and I want to buy from SEARS in Chula Vista, California, but I don’t want to pay double tax (in the US and Mexico) since I am going to import the merchandise to Mexico. Is there any way to avoid the payment of taxes in Chula Vista?

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 a Future Delphi credit scoring and how does it work?

What are the tax changes I need to know about when filing my individual return for 2012?