How many ECB receipts can I use on a purchase? Lets say I purchase $20 worth of items and I have a $10 ECB a $5 ECB and a $4 ECB can I use them all at the same time or do I have to choose one of them to use PER PURCHASE?
There is no limit on how many ECBs you can earn or spend on one transaction.

Can I use my ECB's online?
You can only use the quarterly ECB online, not the ECBs that print at the bottom of your receipt

Do I earn ECB's if I purchase items online?
No.