Offers to buy minerals go out by the ~ tens of thousands per day. 99% of them are deeply to the purchasers advantage, and many of them are simply teasers to attempt to get you to contact them.
As to what is “fair”, this is mostly a nebulous term as used in this context. The right question is, what is the current market value of this. In today’s market, the buying crowd is primarily attempting to purchase your minerals just ahead of future drilling. They have the data, you don’t.
You should know the legal description of the tract of land in which you own minerals, and know what percentage of the minerals you own. There are generally GIA mapping features on the websites of the oil regulatory bodies in all oil producing states.
Dates on which offers ‘end’ are generally meaningless - a simple technique used by marketers to invoke some sense of “you must act or lose out”. Tom foolery.
In the mineral buying industry, there are, many many sitting at desks smiling and dialing - telemarketers trying to get you to pick up the phone. No news there.