Well, let's ask the law of the Church.
Can. 1190 §1 It is absolutely wrong to sell sacred relics.
So, why the donation?
P30 for a small card with a small cloth? I don't like the sound of that
So unless anyone here comes out with a clear definitive statement from the Church that 3rd class relics can be "bought" as long as it is a "donation" and not a "sale", better stop that practice.
The Church law is clear. Relics are relics. 1st, 2nd, or 3rd class, they are all relics.
And the law is clear.
Relics are not to be sold.