Occasionally, SB will offer special bills, with varying SB amounts. Sometimes, it is just 1 bill (like today or on a holiday), other times you try to collect a group of them (like 5 or 6), and if you collect the entire group you get a bonus. It is totally random whether you will get a collector bill or not, so if you don't keep trying. You can earn them just by doing your regular searches. Go to your activity ledger and click on collector bills. This will tell you what bills you need to complete and which ones you have found. Hope that helps!