Safe way to pay an agency that's possibly a scammer? Is Paypal safe?

Good rule of thumb: If they allow you to pay via pay pal they most likely are not a scam. Paypal has a system that allows people to report companies after just a few reports Paypal refuses to work with the company again. I would trust paypal the most, but keep in mind that if you willingly pay for something, Banks, credit cards and other money transferr services are not obligated to ensure you get your money back.