Credits required for Standard advertisements is more expensive than the cost of Fixed advertisements, therefore they only want to use their credits to the members who can afford their products. Best way to filter those people is to choose upgraded members as their audience. it is the general assumption that if a member doesn't even spend $5 for flash clicker program , he/she is less likely to purchase any products online, so why would use expensive credits to members who wouldn't wanna purchase the niches/online products. I know this because I invested a lot of money on this site and I am also advertising my niches on Grandbux and usually i set the daily credit limit to 2000 for non upgraded members and the rest of my credit limits to upgraded members who i get affiliate commission off their purchase. Hopefully this information will enlighten you a little bit.