Hi Max,
you can’t change the wording via a php snippet, but as Stiofan explained, you can edit any default text via translation file, which is easier than applying a snippet.
Basically open the language file (https://github.com/AyeCode/invoicing-quotes/blob/master/languages/wpinv-quotes-en_US.po) with a software like Poedit. Change any text you want and save the file. It will give you a new mo file.
Adding that mo file to the folder wp-content/languages/plugins will modify the default text with your edited text.
This is how WordPress allows to translate website from English to another language, but it can also be use to edit strings that are not written correctly or that are not perfect for your needs.
Here you find the documentation about this of the other plugin of ours. https://wpgeodirectory.com/docs/translate-core/
The process is identical.
I asked to the developers to work on allowing to modify the notifications via a UI like the plugin you mentioned. However it’ll take time before we have this ready.
Let us know how it goes for now,
Thanks