I hope you are well today and thank you for your question.
Where does the text "Please enter your shipping information in the form below to proceed with your order." come from?
The text comes from the following MarketPress plugin file from line number 331
My aim is to change all instances of "shipping" to "billing" as my shop sells digital event tickets. I've changed all the other instances of "shipping" but can't find this last one.
To change the text of MarketPress plugin you can just translate the MarketPress plugin and change its default text by using the information posted on the following page.
Alternatively you can use the following plugin to translate it.
Note: Please don't make any changes in the plugin file and these changes will be overwritten and lost when you update the plugin and you will have to make these again after plugin updation.