You may see this in your STW account logs:
Lock to account: Shared IP requires a Paying account
Referrer already in another account!
This occurs when a shared IP address is already "reserved" by (or locked to) another ShrinkTheWeb Free account. By default, shared IP addresses are blocked in order to prevent abuse by those who have tried to sign up for multiple Free accounts to bypass limits. Using the embedded request method or becoming a paying customer (any upgrade) will override the shared IP block.
Note: We are planning to release a new pricing model, later this year (2016), that will offer a $1.99/month upgrade option just to allow "shared IPs". This is meant to allow those paying for full account upgrades currently to downgrade to the $1.99/month plan and save 40%-80% monthly! Yay! :)
The goal in charging such a small amount, as opposed to just allowing multiple Free accounts to share IPs, is just to ensure that the user is legitimate. Those abusing the system never pay even 2 dollars a month, so this is meant as a deterrent for abuse and theft of free service.
For Free Accounts:
In general, any script that runs in the background will run under a server account that pulls the default server IP, which is typically a shared IP on non-dedicated servers. In other words, we could not even get a dedicated IP to work on a shared server, because the server still used the shared IP as the requester/referrer.
As far as we know, the only option is to subscribe to any upgrade, which will automatically allow sharing of IP addresses.
If the "reserving" account is currently inactive or banned in our system, we will move the IP referrer to the new, requesting account. This happens automatically now. Read more here.