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.