Redline means to highlight changes to drafts. Here, the reference to changes the customer wishes to make to the draft document, which is likely a preliminary wording of an agreement or contract. Red is a traditional color of warning, and the company wants to be able to see clearly the parts of the document with which the customer is in disagreement. You may have confused this term with
strikeout type, which indicates language to be eliminated.
Be aware that the term redline has different meanings in different contexts. For instance, it's the point on a meter indicating danger. If the meter is a tachometer, then the engine is in a dangerous state when the tach redlines. Banks have been known to exclude certain neighborhoods for consideration for mortgages, and this is known as redlining since presumably the banks had maps with the forbidden neighborhoods outlines in red ink.