I understand the hurt, anger and frustration you are feeling, mainly at the dad but getting wound up about a child's expectations and "unfairness" of them is a hiding to nothing I'm afraid.
It won't change the dads behaviour, will make your ss sad, hurt and perhaps feel resented by you and will put a real dampener on Christmas overall.
I get it I really do! My ex never paid cm in full or without being chased a lot and not even always then! I raised dd essentially as a single parent from the age of 2 until high school and then around that time she realised that I had been protecting her from her dads lack of effort (not just maintenance, I bent over backwards, emotionally, practically and financially to ensure she still saw and had a relationship with him, covered for his cock ups until I reached a point I simply couldn't do it any more!)
She asked me to stop doing so, let him sink or swim by his own efforts and the result was within a year all contact had ceased she got a Christmas card the following February with her birthday card! 

She ended up not seeing him or having any contact until last year. She reached out to him (he could ALWAYS get in contact with her if he so wished - he knew our address and landline number and was never blocked by either of us on mobiles or sm but he blocked us - yes he blocked his own daughter off sm)
For whatever reason this time he was open to speaking with dd and they've been in contact since. It absolutely hasn't been smooth sailing.
Dd is an adult now and I can't make her decisions for her. She knows my opinion of him and what he has/hasn't done (which he has of course denied/lied about) and is navigating the situation cautiously and tentatively.
I'm hoping it goes ok but I'm also worried.
All you can do is support your dp and ss
If you can easily afford that amount on a single present and spend similar amounts on all dc (have you any of your own? Can't tell) and it's a reasonable amount in your family's budget then it's up to you and dp.
He (dad) is not your problem. I know it's hard right now, but kids are generally not daft! They figure out what people are really like. I've seen it time and again in such situations.
They may "play nice" and be polite and civil to the less than stellar parent but they are usually pretty good at sussing out not to rely on them.
They also do learn to show their appreciation for their resident parent and step parents.
I have several friends my age who had step parents, and on occasion were little so and so's in childhood/teen years to them but as adults they're very close and very much appreciate all the step parent did for them.
It's a long game and no it's not easy at all!
You didn't come across as a dick to me you came across as hurt and worn down which isn't surprising considering the circumstances, the season AND the tough year we've all had!
Don't be so hard on yourself.
Hopefully will make you laugh. She is very wary to even disagree with her dad on minor matters. but she will quite happily argue with me until the cows come home! She'll even call me JUST to have a cathartic arguing session if she's having a stressful day! Real humdingers! 
I take it as a great compliment as it means she feels totally safe and secure that NOTHING she can say will make me not love her - she doesn't feel that about her dad at all.
Love is what makes a parent not biology