Subtle, but this is spot price - typically the price negotiated by large institutions based on large transfers. Individuals in the $300 range of silver buying pay premiums on top of that which scales down with size; a single gram bar has a higher premium and costs a lot more than a single 10 ozt bar with a lower premium. This can be observed at any precious metals trading platform, whether it's corporate sited like APMEX or community sites like reddit or dedicated forums; it would be highly inefficient to buy gram bars to fund anything - but people with $300 in life savings probably can't afford more than 1 ozt at a time. Silver spot price is pretty volatile as well, the trade tariff war has driven it up $2 or more in the last year.
You could also read the paper. The part linking silver prices to charitable giving is straightforward.
Second, try to calculate the silver price fluctuations, and calculate the ACTUAL dollar amount that becomes disposable to spend. Its peanuts.
Finally, this category of people do not give charity. They take charity.