In addition to monthly subscription billing, Shopify also issues bills throughout the month according to your plan-based billing threshold. Billing thresholds are dollar values that trigger billing whenever the cost of your account activity exceeds its threshold amount. You can see an example of how a billing threshold works below:
And read more about it in the help guide Shopify has for it here.
Hope this helps!
Would you be able to go into a little more detail for me about this, please? Can you let me know what you mean by an issue? Is something not working correctly in some way?
You can view your billing/ threshold history in your admin > Settings > Billing > Bills > Recent Bills also if that might provide some clarity for you. A very common reason for threshold billing is via Shipping labels. Shopify has a great guide that goes into some detail about threshold billing which you can read here.
If you would like to talk to someone on our support team in real-time about this you can do it from here, logging in and following the steps.