The primary reason why a lot of SaaS businesses charge a monthly premium instead of an annual premium is due to purchasing policies at corporates - which usually dictate that purchases over x amount (x=$500/$1000+) require multiple sign-off and follow long-winded acquisition processes that tend to take forever. By charging less than the threshold or by charging a large amount that would normally exceed the threshold over time; it's a lot easier to get such purchase orders signed off. Credit card is the preferred method for smallish amounts (< $2000).
Without generalizing too much - corporates are generally price insensitive - they would rather pay a preferred supplier more than risk their business on a cheaper alternative (without great motivation).
It's in your best interest to dictate the terms that are the least risky for you - they won't choose another supplier because he offers payment via cheque and you don't. If they do, I'm not sure that you do want them as a client considering the hassle you would potentially have to go through to get your money (good luck if the corporate is in another country!).