How are the invoices calculated when a subscription is upgraded?

If you decide to upgrade your package halfway through the month, you will not be charged the full extra amount.

For example, if you are on the 60 submissions package for £19 per month and decide to move up to the 250 submissions package for £59 per month, you will not be charged the full amount for the new package. 

This is because part of the month has already  been and gone and so you probably won’t need the full amount of submissions. The balance you will have to pay is the difference for the time remaining.