In this passage we see differences in grammatical number for three instances of the noun labor, beginning with the plural labors, switching then to the singular labor, and ending up with the plural labors (labeled above as 1, 2, and 3). In the third instance, Oliver Cowdery initially wrote labor in 𝓟 (spelled as labour); sometime later he added the plural s (the level of ink flow for the inserted s is somewhat heavier).
In general, the text allows for either singular labor or plural labors. See, for instance, the discussion regarding the noun phrase “the fruit(s) of one’s labor(s)” under Alma 29:17 or the verb phrase “to perform one’s labor(s)” under Alma 53:1. In some cases, we get only one of the two possibilities, such as the plural in “labors of X” (see the discussion under Mosiah 7:16). Given the phrase “for one’s labor(s)”, we have here in Alma 30:33–34 both a singular and a plural; in Mosiah 18:26 there is one more instance of the singular: “for their labor they were to receive the grace of God”. For each case of labor(s), we will let the earliest textual sources determine the grammatical number.
Summary: Accept the textual switching between the singular and plural forms of labor whenever they occur in the text, including the switch from labors to labor and then back to labors in Alma 30:33–34.