Is it ok to implement minimum purchase requirements for credit and debit card transactions? The short answer is: It depends.
Let's start with credit cards...
Credit card purchases may in fact be subject to minimum spending requirements. A federal law passed in 2010 gives merchants the right to require customers spend a minimum amount (up to $10) when paying with a credit card. The only condition is that you have the same minimum purchasing amount for all card types (Visa, MasterCard, Amex, Discover, etc).
As for customers paying with debit cards, while it isn't technically illegal to require them to spend a minimum amount, there's a good chance that doing so may violate the terms of your contract with PayAnywhere. Note that credit card companies encourage customers to report businesses that require debit card purchase minimums. Such a report could also result in you violating your contract. Make sure that you read all of your contracts carefully and understand the terms within them. Don't hesitate to ask questions if you don't understand exactly what a part of a contract means and you can even reach out to an attorney to help you understand exactly what is within a contract.