Doxycycline is used for malaria prophylaxis or treatment. When it is administered for treatment of P falciparum malaria, this drug must be used as part of combination therapy (eg, typically with quinine or quinidine).
If you buy goods and are not charged the Rhode Island Sales Tax by the retailer, such as with online and out-of-state purchases, you are supposed to pay the 7% sales tax (less any foreign sales tax paid) for these items yourself as the Rhode Island Use Tax.
Are shipping & handling subject to sales tax in Rhode Island? You will definitely have to part with shipping charges once you are in Rhode Island. In most cases, they consider shipping charges as part of the sales price of the item. That means that, if the item is taxable then shipping is taxable too.