If an invitation to any event makes you feel uncomfortable, irritated or frustrated then the answer is simple; just don’t go. You can politely have “something else going on”. By not going you shouldn’t feel the need to send a card/money in your place. You’re busy, you can’t attend and so you won’t be providing a gift, oh well. I wouldn’t get offended if I invited someone to an event and they “couldn’t make it” and didn’t send a gift. There are too many occasions for too many people to make everything anyway and everyone should understand that.
With that being said, I graduated from high school a little over a decade ago and it was nothing short of a miracle. The only thing I had achieved in high school was being named “Class Clown”. I was proud of myself for graduating high school and knew that I would be about to begin the journey of the rest of my life and I saw my Graduation Party as a way of celebrating what I had accomplished and a way of seeing everyone before I headed off for Lord knows where I was going. Yes, I certainly didn’t mind that my guests gave cards/money, but at the same time I felt almost awkward receiving them. I really did only want to share the day with those people, I was much less concerned about the money. I had friends of equivalent ages attend my party empty handed and I was just as happy that they came. For the record, the couple of hundred dollars that I ended up with put my first car on the road that got me to and from college, so at least it contributed to my college education.
On the other hand, I can absolutely relate to what you are saying and sometimes I can do nothing more than raise my eyebrow to some of the invitations I have received over the years; I think, “Do I even know who this person is or do they even know me?” What is the point of inviting near-strangers to a joyous personal event? Yes, I do think that oftentimes, money is the main motivator behind invites, not necessarily the party itself but probably half of the people invited.
Like I said, just don’t go but express your congratulations.