Whether donation is the same as fee really does hinge on whether people truly would get the weddding service if they declined to pay the donation. I'm not convinced.
'Suggested' levels of donation imply otherwise or the church would just ask for whatever a couple felt they could personally afford to contribute (if anything).
Saying you're "not convinced" is really just another way of saying that I must be lying, isn't it? Implying that someone is lying just because they don't agree with you, and when the matter under discussion is one they have greater knowledge and experience of, surely should be recognised as a specialised area of CF-ery.
I am a minister. I conduct weddings. My denomination (not CofE) explicitly forbids me from charging a fee to conduct a wedding. I do not accept any money for my time in planning or conducting a wedding. That applies whether the couple are church members or not.
It is usual for churches to have an idea of how much it actually costs them to have a wedding, or any other service, in their building. This will vary from church to church and may include energy costs for heating and lighting, cleaning costs, etc. If there is a "suggested donation" then this will normally be based on the actual costs incurred by the church.
Also, it's not unusual for people to ask for some guidance in what would be considered a reasonable donation. Being able to give them an idea of an amount they might choose to give, based on actual costs, can be helpful for them.
A donation is a gift. It is freely given, and may not be given at all. I can assure you that any person who comes to my church for a service be that a wedding or a funeral, will receive exactly the same level of care, attention, and service irrespective of how much they give.
And, just in case you think I might be subconsciously swayed by the amount given - any donations are received after the service, not before. And even if I did receive the donation in advance, or know in advance that there would be no donation, that would make no difference to me. You may well differentiate between people based on financial considerations, but I do not.