Quarters minted before 1964 are 90% silver, but even then the total weight is 0.1808 troy ounces per quarter. A quarter would have to be over five times as large in order to be composed of .9 oz of silver.

Now if sovcit wanted to do the bank right(!) here, he'd attach a .999 troy oz silver coin, such as a US Silver Eagle, which would set sovcit back about $40 or so. Postage would increase, if sovcits pay postage, because the envelope would need to be padded and the increased weight of the coin versus the quarter would increase rates. I did not do the math on that - someone else can.

No, no, you gotta think sovict.

Its a coin, that is silver in color, i.e a "silver coin," that is .9 0z.

No, what the sovcit attached is called a bullion coin. They usually have a nominal value, so they are official legal tender and you can pay with them in a store, but their actual worth is basically the price of the precious metal they are minted from (+collector's value for some coins) which is usually much higher. 0.9 troy ounces of pure silver are around $28.

It is the coin holder's duty to liquidate the coin before payment, so the sovcit basically just scammed themselves and gave their coin away for the nominal value (although it will likely be returned as this letter will not be accepted for obvious reasons).