You can use the add-on free of charge, however there is a limit and you can generate 20 documents in a row. Then you need to wait one hour to reset your quota. Furthermore, each document contains a link to anymerge.com in the header and in the footer.
Use the free quota to experiment with the add-on to see if it suits your needs and can help you automate your manual tasks.
Once you get more comfortable with the add-on you can purchase credits. For one credit you can create one document. This option is ideal if you need to generate documents from time to time. Obviously there are no more links to anymerge.com in the documents.
Finally, for frequent users who need to generate documents on daily basis there is a thirty day pass available. If you need to generate larger quantities of documents at once there is a one day pass available as well. Once a pass is active you can generate as many documents as you need*.
NOTE*: Please keep in mind that Google imposes its own quotas for number of documents you can create per day. For standard Gmail account it is 250/day for business accounts it is 1500/day. More details.
NOTE: You can combine credits with subscription. Subscription always takes precedence. If you have bought credits and you would like to switch to subscription you don't have to use all paid credits but you can activate subscription anytime. Once your subscription is over add-on will revert back to paid credits.
NOTE: Also wort mentioning is that by activating new subscription your active subscription is extended. For example if you have last ten days of subscription and you purchase another 30 Day Pass your subscription will be extended by thirty days and you can use the add-on for another forty days. The same holds true for credits.