Hi,
I received payment from 500px and was charged a fee by Paypal (about 4%). I contacted 500px and they say they did realize payment via Paypal mass payment so there should not be any fee. I encourage you to look at your Paypal account and see if you are not charged for something you should not be. Please let me know if this is normal for 500px Paypal payments.
It happened to me once, when I accidentally mixed-up my Paypal address.
I missed their regular free mass-payment and, after getting in touch with them, they sent me a personalized payment, charged with Paypal fees.
I payed for my mistake, so... no complaints.