Yes, it is possible.
If you are a "Business" plan user, you can choose between a "Subscription Purchase," which allows you to change your monthly sending limit, and a "One-time Purchase," which allows you to purchase additional documents on a temporary basis.
Option 1）Subscription Purchase :
Additional documents will be available for purchase as a subscription (new purchase method), which would increase the monthly document limits.
*Additional document subscription periods will be the setup to match the base plan (Business) subscription period
*Cost: 100 yen per document (can be added in increments of 50 documents)
Option 2）One-time Purchase：
Additional documents will be available for purchase as a one-time purchase when additional documents are needed for a specific month. (Same as current purchase method, cost changes)
*Cost: 200 yen per document (can be purchased in packs of 10 documents)
*If you are subscribed to the Entry Plan, Standard Plan, or Premium Plan, please contact support@signtime.com to purchase additional documents.
*Purchased additional documents expire monthly (monthly date of your contract renewal). Expired additional documents cannot be used the following month.
*For details, please check How to add the number of documents sent.