I went to "MID POINT COFFEE" with my co-worker.

It has a discount event each day.
Revised: Everyday there is discounts on drinks.

For example, on Monday we can buy a cup of hot chocolate(M) by $2.99(+TAX).
NOTES: "For example" isn't wrong, but it's not needed here.
REVISED: On Mondays, we can buy a Medium cup of hot chocolate for (not by) $2.99 + tax.

Usually they put some fresh cream on it but in my case they suggested me "Do you wanna add some marshmallow?"
Of course,I was happy!!
REVISED: Usually they put some fresh cream on it, but today they asked me if wanted to add marshmallows to it instead. I said "Of course". I was happy to do that.

However,I forgot that I don't like a sweet drink again.(I usually drink black coffee)
Although it was tasty,I added a lot of skim milk to decrease the sweet.
I wonder why I orderd it.haha
REVISED: However, I had again forgotten (past tense) that I don't like sweet drinks (I usually drink black coffee). It was tasty, but I had to add a lot of skim milk to it to decrease the sweetness.