I have always manually set up hosting accounts for my customers, but this system has its limitations. In today's fast-paced world, many customers expect instant setup, which manual setup cannot always provide. I have found a solution with 20i.com, who offer a free 'Hostshop' with automated package and product setup.
but, in order to use this service, I would need to invest in domain credits, which I currently cannot afford. I am curious how other businesses handle this issue, especially those who are not yet able to invest in domain credits. Are there still manual options available?

You can use a variety of billing systems such as Blesta, WHMCS, clientexec, hostbill, your own code, or even Bill and Ted's Awesome Hosting Billing System. If your reseller account has an API, you can hook up the billing system to it using a module. For control panels like cpanel, plesk, or directadmin, there are systems available that can connect to the API and automatically create new accounts for you.
This applies to general reseller hosting and hosting in general using those panels, although I am unsure if this is applicable to 20i.

It is crucial to be cautious of fraudulent orders when implementing automatic account creation, particularly if the system will purchase something on behalf of the new customer, such as a domain.

This is necessary to prevent unauthorized access to your servers by malicious individuals who may use the account for spamming, phishing, or exploiting vulnerabilities. The risk associated with accepting orders using stolen credit card information is too high, and chargebacks can occur long after the damage has been done.
While rare, some customers may encounter issues with this process, but they usually understand the rationale behind it once it is explained to them.

When a Plesk hosting subscription is created, a system user is also created in the Plesk web server's operating system. Customers receive their system user credentials from their service provider, which they can use to connect to the server via FTP or SSH. All file and folder operations are performed by the subscription's system user.

In previous versions of Plesk, it was inconvenient for users with multiple web hosting accounts to access the settings for individual domains or mailboxes. Plesk now allows customers to easily navigate among multiple subscriptions and display all websites and email addresses on one page. Sites from the same subscription share a system user.

New hosting server resources can be added to an existing subscription without purchasing a new subscription, such as adding SSL support or additional disk space.

If a subscription has been suspended, customers cannot manage any aspect of the subscription, and their sites may be unavailable. Hosting providers are the only ones who can renew subscriptions, so customers should contact their ISP as soon as possible to resolve this issue.