Here is a fine way to conclude your letters.
Thank you for your thoughtful gift. May God reward your kindness.
Oremus pro invicem,
In The Latin Letters of C. S. Lewis, the authors frequently end their letters in this way. Oremus is from oro,orare, to pray. It is in the subjunctive mood, and in the first person plural, which renders it Let us pray. The word Oremus always precedes the collect and the post-communion prayer at Holy Mass in the Novus Ordo Missae.
pro = for. The word invicem can throw you off because a cursory glance may lead to the incorrect assumption that the em indicates accusative, but in reality this is a non-declining word meaning (from Lewis and Short) one another, each other, mutually, reciprocally. Hence, “Let us pray for one another.”