Adverb: Inexpensively, reasonably, advantageously.
Edullisesti is formed from the adjective edullinen (”cheap”, ”inexpensive”, ”economical”) plus the suffix -sti, which can be used to form adverbs from adjectives. The suffix is appended to the weak stem, i.e. take the ”n” off the genitive singular form and adds this suffix.
Edullinen can be broken down further into etu, a noun which means ”advantage” or ”benefit” plus the suffix -llinen, which can be used with nouns to create adjectives indicating possession of a quality or likeness to it. Again, the suffix is appended to the weak stem.
Let’s translate the whole ad, ”Vuokraa autot edullisesti ja vaivattomasti,” ”Rent cars cheaply and easily.”
”Vuokraa autot” means ”rent cars.” Vuokraa is the 2nd person imperative form of vuokrata, which means ”to rent” and autot is accusative plural of auto, meaning car.
Vaivattomasti is an adverb that means ”easily” or ”effortlessly.” It comes from the adjective vaivaton. Here, vaivattoman is the genitive singular, from which we remove the ”n” to get the stem and then add -sti.