If the App Store or Google Play offers a free trial or introductory price for Magpie Pro, you can now buy Pro with that offer. Before, the buy buttons showed an error instead.
The plan screen shows the offer before you buy, for example "Free for 1 week, then $4.99 each month". It also shows the full price that renews after the offer ends. On iPhone the offer is shown only if your Apple ID can still use it. Otherwise the screen shows the full price.