How do you politely tell someone their a douche bag?

As part of my multifaceted new years resolution to be an overall better person and make my life easier, I've made it part of my plan to not be so ....harshly honest with people. Maybe stay true to my beliefs but just be a little gentler about them.

For example: Say someone owes you a mighty fine sum of money. So you agree to take it out in trade. So while they do you a favor, they decide that they still feel bad and want to help you out with a smaller sum of money (in addition to said favor). Well...turns out I'll get that out to you this month doesn't actually mean within the next 30 days. In fact, it doesn't mean within the next 4 months!  But that's okay whatever, I was willing to call it square with the favor. THEN! You get a invitation in the mail to their wedding shower...seriously man?? You screwed me out of money, didn't follow through on your word...and NOW you want me to buy you a present? (ballsy) Then, you receive an wedding invite...you know what, bygones...no worries. You say you'll be there. Oh hello, what's this? I have an email...Please go to the tuxedo store and RENT a tux. Oh and don't forget, the night before the groom wants to have a poker night, please be there. Wait, what?? I'm sorry, did you just tell me to rent a tux? Ok, let me TELL you all the reasons why THAT's not going to happen. Ok, A: you know I don't have tons of money to be spending on living much less YOUR wedding. B: you want me to take time off work (which of course is the source of my money)....

As it stands, we ended up not having the money for the gas it would take to drive an SUV up and back. Fun times...who knew that having the gas to get to work would be more important. I don't wish them bad luck, but more I wish COMMON SENSE upon them.