Psa is the least used of the twenty-six. In classical Ge'ez it appears chiefly in loanwords and in a small number of names, and a reader can work through a good deal of liturgical text without meeting it. Payt, the other p letter, is commoner, because the names it spells, Pawlos and Petros among them, are constantly in the lectionary.
Rarity of that kind is worth recording rather than glossing over, and it has a practical consequence: a letter that a scribe writes rarely is a letter whose form varies more from hand to hand, and it is one of the first places a palaeographer looks when dating or localising a manuscript. The letters a script uses least often are frequently the ones that carry the most information about who wrote it and when.
The modern languages have changed the balance. Amharic and Tigrinya take loanwords with p from European languages freely, and the letter is now ordinary in a newspaper where it was a curiosity in a psalter. That is a reminder that the fidel is a working script and not a closed classical inventory. The atlas records the twenty-six letters of the classical Ge'ez alphabet, which is a defined thing with a fixed membership, and the wider Ethiopic script that has grown from it is a larger subject with its own history still in progress.