How to implement the checkout form?

Question

Online shopping

Colleagues! I work as a project manager. Periodically, we have to develop online stores, and in general, many mechanics of working with users have already been worked out to automatism, and we use them in almost every project.
But there is one case, which so far is being solved differently on different projects. I describe: the user is already registered in the store (the identifier is his email), and proceeds to place the next order. At the same time, he forgot or did not want to log in to the site, and in the order form he drives in his email registered in the system.
As a rule, in the case of placing an order by a new user, we automatically register it in the system, immediately authorize it and transfer it to your personal account. At the same time, a password is sent to the mail to enter the system (yes, I know that the password in clear text is bad). And so, in the context of the case described above, a complex dilemma arises. What solutions did you try:
1. Highlight the email input field as an error, and explain to the user that he needs to log in before he continues. This option has the right to life if the email field is the first one and the user has not had time to fill in hundreds of other fields.
2. Throw out the authorization form with the email substituted there. The difficulty here (and in the previous paragraph) may arise if the user has forgotten his password - a lot of time and user actions may pass until he continues to place an order (and most likely he will score and leave the site).
3. Place an order anyway to this email, but do not authorize the user. The minus is obvious - I can, knowing any other person's email registered on the site, send him as many new orders as I like. Pampering, of course, but it is unpleasant that such an opportunity exists in principle.
4. In the order form, always display the "password" field. If the user is new, he registers, if the user is existing, then he logs in, unless, of course, he remembered his password.
5. Even before placing an order, issue a form in which the user will have several options to choose from: "I'm a new user", "I'm already registered" (and then he fills in the email and password) and "continue without registration". But after all, the user can say that he is new, and drive in an existing email, and back to the original question :)
All of these methods have their pros and cons. So far, I have come to the conclusion that the most painless option is number 3. I would like to chill a little, suddenly the truth in the dispute will be born (or maybe I just don’t know some obvious way to solve the problem, and someone will set the right path ).