You were promised this item, and if the rep is having a bad time with his chemo, he clearly has colleagues who should be helping him out by going the extra mile to see his work gets done.
They have not just played the cancer card. They have used it to explain why they have failed to step in to help him when he understandably dropped the ball. They have let you down, and him down as well.
And they have made you sad that you failed to have the present on time after all the effort you put into it.
Also, I find that cancer treatment does take it out of me and makes me more likely to get upset about things I cannot do anything about. Hitting my head against a brick wall is a sign I am carrying too heavy a load and need a bit of kindness, maybe from myself.
And sometimes I need to write and post a complaint letter, or put a review online about the company in order to feel I have actually done something before I am happy to back off and let it go.