The Transmission logo used on the PayPal page uses http instead of https. This means that Safari doesn't show the padlock, and Chrome shows a warning sign instead of a green stamp/seal icon (or whatever that is).
I thought that somehow I may have been phished or something, or maybe the site was compromised to link to a sketchy site that looks like paypal.com using Unicode homoglyphs (a term I just made up for characters that look the same but have different code points).
I'm going to donate anyway because it looks legit. The only http resource I found on the page was the Transmission logo. It's possible that you are losing a small amount of donations over this though, and I bet every bit helps.