I stumbled upon this account in Fr Schouppe's Purgatory, and I consider finding it to be an answer to prayer because I had prayed for the truest understanding of the nature of indulgences, and this is supremely accurate because indulgences are treasures and a form of wealth that we may use to pay the debt owed on our sins - and the sins of others - and the sins of the Holy Souls. When you or I gain an indulgence for ourselves we remit the temporal punishment due to our sin. I had thought of an indulgence as a grace that is like a cleansing water on the soul which cleans away the sin, thus rendering redundant the need for temporal punishment, but this is very inaccurate because it is in the Sacrament of Penance that we are absolved of our sin, but when we go about getting indulgences (such as when we make a visit to the Blessed Sacrament) we pay off the debt of temporal punishment that is owed after we have been absolved.
But this practice of gaining indulgences wherein we pay off/remit the punishment due to sin is the sole purview of the living, the prerogative of us who are alive. The Holy Souls may not gain indulgences for themselves. We, however, may transfer an indulgence to the Holy Souls, or to a particular soul, and we earn a ruby or a pearl that we offer Our Lord as the price paid in lieu of their punishment. I am loathe to give examples of how one may gain an indulgence (such as offering a Rosary, doing The Stations of the Cross) because there are so many, many ways to gain indulgences that to mention examples is to risk giving the idea that only the pious devotions listed are the means of gaining an indulgence. Thus it is best to buy The Raccolta. The spectacular import of Our Lord's message to Blessed Mary Ann is that every living soul may approach and take as much as he pleases, which means for you or me is that we may take as much as we want from this buffet of indulgences.
Personally, I do not gain indulgences for myself. Some years ago I made an act of my will to give all my indulgences to the Holy Souls (even if at the time I didn't understand indulgences as a treasure trove of gold and precious gems). The reason I deigned to give away all my indulgences is because the plan of dividing my indulgences into those I keep for myself and those I allot the Holy Souls did not sit well with me, and to have peace in myself I offered all to the Holy Souls who are in such dire necessity.