A 29-year-old G1P1 presents 2 hours after vaginal delivery with heavy vaginal bleeding. Estimated blood loss is 1200 mL. Blood pressure is 85/50 mm Hg and heart rate 120 beats/min. The uterus is boggy and enlarged on palpation. According to SOGC postpartum haemorrhage guidelines, what is the most appropriate first-line uterotonic therapy (in addition to resuscitation and uterine massage)?