The script is sold by OSBot, I can't accept payments myself. But there is an alternative to paypal in the store, credit can be obtained by redeeming vouchers. These are sold by many people on the forums, they accept most payment options including osrs gp, like this one
The script computes prices based on live GE data from online sources (eg rsbuddy exchange api), prices change every time those sources update, I think that's somewhere in the 15-30 min range