If I received commission from a company for sales I performed ($100), but gave that commission back to my customer as a discount (-$100), what are the tax implications on me as my net income is 0 after the discount? Do I still pay tax on the full commission I received?

No but you still need to report the commission income of $100 on your return however, as you give it back to your client as a d/c, you may subtract it as sales returns or deductible expense maybe as other expense on your Sch C of 1040 and then your taxable income is zero no need to pay tax.