The biggest difference between the two pricing structures is the features! With the monthly plans, you get the recipient portal, which is the personalized URL you will create upon registering [https://yourcompany.simplecert.net]. This URL is where all your recipients can go to create a free account and download all their certificates from you in one place. The other big difference is that each has credits. However, Pay as you Go (PAYG) credits don’t expire and are a one-time payment. Monthly Plan credits replenish every month (on day of sign-up).
PAYG credits are found in the plans menu underneath the monthly subscription plans. There are buckets of credits avaialble and you may them in multiple buckets for the 1000 credit option.