This has been a point of confusion and frustration, so I wanted to explain in detail how this will work from now on.
Previously, if you purchased a subscription before the end of your free trial sub, the paid subscription kicked in immediately (because you'd want to get the stipend right away) but any time remaining on your free sub went into the ether.
Now if you buy a subscription with time remaining on the free trial, we'll pay out the remaining time at a rate of 10 credits a day) — if you pay for a subscription without ever activating your free trial, you'll receive a full 300 credits.
The bonus credits will be shown on the post-checkout confirmation page when the payment completes (unless it's a pending payment, in which case the refund is given when it is finalized). also show up on your credits history page.
And, this applies retroactively, all the way back to when beta subscriptions were first offered. So, many of you will be finding a nice little bump in your credit account. You can check if it applies to you by looking at the credit section of your account history.