additional purchase
noun phraseUK /əˈdɪʃənəl ˈpɜːtʃɪs/
US /əˈdɪʃənəl ˈpɜːrtʃɪs/
Something extra you buy besides what you already got.
The toy came with batteries, but the charger was an additional purchase.
OTHER MEANINGS
2. An extra item bought to go with a main product.
The phone case was an additional purchase at checkout.
3. A second thing you pay for after your first payment.
Tickets were free, but snacks were an additional purchase.
4. Something bought later to complete a set.
She bought the bike, then made an additional purchase of a helmet.