CVS Spend $X.XX, get $X.XX using EB? Newbie Question

Super new to the couponing world so bear with me. Can I use EB to "spend" a certain amount to get EB back? For example this week's Colgate spend $8, get $4 in EB, I want to cover a portion of that $8 with EB.

Also I'm concerned about getting 2 Colgate toothpastes for this deal because they were originally $3.99 each, and they are now currently $3.79. Obviously this will total out less than the $8 required... Does CVS take the original price or sale price for these Spend X, Get X coupons?

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Thank you for reading and sorry if these are dumb questions!