Waiting multiple days to buy credits, causing my website to suffer

So I own a fairly large website that gets a lot of registered users. I use transmail send activation emails, and also forgot password emails. I sent an email to zoho's presales team when I was only at 4K/10K emails sent, hoping to buy more credits before I hit 10K. It has now taken multiple days and I have completely run out of emails, and my website can no longer send emails. Zoho has responded saying they would send me an invoice, but 2 days later, I still have not received an invoice. Is this really how long it is going to take to get more credits? Should I be relying on Zoho transmail for this type of website, or is this a bad idea? I really like Zoho transmail as it worked very well... until I needed more credits.

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