Shares need to be “fully paid” before they can be issued. Shares can be considered “fully paid” after an investor has paid, this does not necessarily mean the funds need to be in your account. They may be in an account held on your behalf, or a wire may be in transit, for example.
So long as the funds have left the investors account, this can often be sufficient for you to proceed with the closing knowing the money is being processed on your behalf and will be sent to you. For online payments you can take the position that because it is in your online payment processor account, this is sufficient. The funds do not have to physically be in your bank account when closing. This is a discussion you may want to have with your counsel.
You will want to consider if the funds are revocable, and what would happen if the funds did not arrive. For example, if the stock is immediately trading, you can chase the investor for payment as a receivable if for some reason the payment was not received, in order to determine if funds are sufficiently submitted to be “fully paid”.