You need to think for this on the basis of the price of an individual item in your service. For example, let's say you charge $30 for creating an infographic, so you need to charge accordingly to the client if they ask for extra deliverables. But, you can go for a discount if they order in bulk.

You are absolutely right. Things should be clearly documented to avoid conflicts of interest in the future. Anything not written down alcan easily be discarded but written down documents can easily be referred to.

As a creator, it is imperative to always have a contract in place so both parties are aligned with expectations on deliverables, timeline and pricing etc. If it is within your bandwidth/timeline to provide the extra services but require a price adjustment, you can ask for a conversation to revisit the terms of the agreement. If you're feeling more flexible, you can clarify you're open to accommodating the request, and clearly communicate how much more time you'd need, but moving forward additional requests would need a price adjustment. If pricing isn't a concern but time is, make sure to get a sense of what they're open to prioritizing vs be more lenient on so you can work within the set boundaries.