50% more
No, not at all. In this case, the addition of 6 ounces to 12 ounces produces a total of 18 ounces, which is a 50 percent increase. That wasn’t so difficult, was it? Apparently, for some people, adding and subtracting six zeroes from a hundred is rocket science.
We hope this individual didn’t return to the shop to voice their complaint, since he would appear quite stupid. Just sit down and enjoy your mustard with a nice cooked sausage; don’t let the burden of arithmetic ruin your dinner and deprive you of all that extra mustard you have.