It is worth stating plainly that "do ut des" is a modern scholarly formula rather than a phrase Romans themselves used as a technical religious term, coined by later classicists to summarize a pattern they read consistently across Roman ritual texts, vow inscriptions and prayer formulas. Historians of Roman religion, including Mary Beard, John North and Simon Price, caution against letting the label flatten Roman religious life into pure transaction, since the same sources that show meticulous contractual phrasing also show genuine anxiety, gratitude and awe running alongside the formal exchange, and a Roman making a vow at a moment of real personal danger, a difficult childbirth, a storm at sea, was almost certainly not experiencing the moment as a cool business negotiation even while phrasing the request in exact, quasi-legal language.
What the concept usefully captures is a structural feature rather than an emotional one: Roman religious obligation was discharged through correctly specified performance rather than through professed inner faith, which is precisely why a technically flawed ritual, however sincerely intended, still had to be repeated, and why the entire system of prodigy, expiation and pax deorum could function as a coherent whole rather than a collection of separate devotional gestures.