From the

Answer:

From the details you have provided, this seems to be a predatory publisher. These are bogus publishers seeking to exploit unsuspecting researchers typically in terms of money with the promise of quick and easy publishing. The indicators in your query are the initial spam mail (a mail from a reputed publisher would typically come to your inbox), the lack of response until acceptance, the response only after submission, and the acceptance mail accompanied by the invoice for a high publication fee. For more indicators, you can refer to this checklist for identifying predatory journals: 10 Point checklist to identify predatory publishers

However, although you may not have been aware of this dubious practice, you took a lot of positive actions throughout. You made an initial inquiry about the submission and charge. You wrote only a small manuscript as a start instead of a complete research paper, which would have involved a lot of time and effort that would have eventually been fruitless. Finally, even at the time of submission, you explained quite clearly that you were expecting the manuscript to be published free of charge. So, good show there!

Here is what we would suggest doing now.

Write to the journal stating that you would like to withdraw the manuscript for the following reasons:
- The publication fee is (too) high.
- You had earlier expressed that you were expecting the manuscript to be published free of charge.
- As the journal hadn’t replied to your earlier mail, you were indeed expecting that it would be published free of charge.
As predatory publishers are quite coercive, they are likely to use various tactics to force you to make the payment. However, maintain your stance of not making the payment and wishing to withdraw the manuscript. As a last resort, they may go ahead and publish the paper and then make another attempt at asking you to pay, now that the manuscript has been published. As before, maintain your stance.
However, in case they do not reply, write to them again stating that if you do not hear from them within a certain time period (for instance, 10 days), you will consider the manuscript withdrawn.
Maintain a record of all communication with the journal for future reference. In case you have deleted some previous ones, that’s fine; keep all mails from this instant onward.
If needed, you could seek to have the manuscript published in some other journal. However, as you had written it only for this journal, that too, as a letter to the editor or an editorial, it may not be relevant for some other journal, and so, you may let it be.
Learn more about the practices and fallouts of predatory publishing. If a researcher publishes in a bogus journal, mistakenly or otherwise, it may impact their academic credentials.