For Prime Members
Amazon offers Prime members the Charmin Ultra Soft Double Roll 10-Count 4-Pack for $20.29 with free shipping ($0.51/roll). Go to the product page, clip the 50 cent coupon below the product to bring it to $21.37, and choose Subscribe & Save for more savings, down to $20.29.
Then, pay with a MasterCard and use code MSTRCRD1 to get another large discount to bring it down to $18.13. Any combination of these discounts can be used for savings if you don't want to use any particular one.