I think I found a way to insert html for Template:Support-large. I'd install this extension and insert the wepay html into a file on the server, as it specifies. It seems to be the most secure way to enable html snippets. Does that sound ok?
Wow, I don't know how I missed that extension, since it has been around for years. Do you want to come over to my apt/office this week to install Extension:HTMLets and do some admin updates?
I think we had/have it as part of Extension:Widgets. But it either stopped working or the embedded wepay code itself stopped working. Either way, since we may be uninstalling Widgets, it makes sense to install HTMLets and use it instead, I think. I'll be there today at 7 if you like.