You don't have to worry about exceeding your account limits. All replies that exceed your monthly reply limit will still be stored safe and sound on our database, as well as all file uploads that exceeded your monthly limit.
All exceeded replies and file uploads will become accessible when there's more available monthly limits on your account. This means that if you're on a Free plan, your account will have 50 of the oldest replies, and 100MB of the oldest file uploads available.
If you would like to increase your monthly limits, you can always opt into our Pro plans. If you're already on a Pro account, you can upgrade your monthly limits as explained in this support article.