Some of us like to have a series of Gregorian Masses said for our deceased friends and loved ones, who may be in Purgatory. This is a consecutive series of Masses said on consecutive days. The cost is obviously higher than just one Mass. If we have these said, we can have confidence that we have done our best for that particular person, although of course whether the Masses provide sufficient grace to get the person from Purgatory to Heaven is up to God, and also it’s possible the specified person might already be in Heaven, or in Hell, in which cases the Masses are used by God to help some other soul.
Missions and monasteries frequently offer this service as a way of raising funds for their own upkeep through the stipends they receive. It is generally not practical for a parish to do it because a parish priest probably has a lot of parishioners requesting that a Mass be said and it is not easy for him to reserve a number of consecutive days for one person. So such requests are usually sent to a mission or monastery, where there are more priests who need to say one Mass a day and may not be deluged with requests for intentions, and can also use the stipends for their support.
I have used CNEWA (Catholic Near East Welfare Association, CNEWA.org) two or three times for Gregorian Masses, for my father-in-law and for a deceased friend or two. As stipends go, theirs is among the lower ones suggested for this service; I tend to spread my Gregorian Mass donations around to several different places, so I see different stipends. They have their process set up very efficiently as well, and I believe you can make all the arrangements right on the Internet without having to call someone or mail a check.