This might be more of a pain then anything else, but...
What if we had a new credit shop pet. ONLY obtainable once-- Whichever month it came out for only a month's time.
Say for only one egg it costs $3 dollars per egg. Yes that is cheaper then the current ones, but they would also only come out that one month, not in a repeating cycle.
Of course the pain of this would be a new pet every month to be drawn. This could even be done say every 3 months. One new pet say in January only. Then another in March/April.
It could also only occur twice a year, Just trying to figure some ways to get more money into the site, while also keeping pets rare and hard to obtain.
I doubt someone will only want to buy one egg, so they will more then likely buy a few of the eggs (I know I will)
Just let me know what you think.