Compare the bundle price with buying only the items you need separately. A lower price for all the parts can still cost more than the original purchase task.

In this invented offer, the notebook costs $20 pens $10 and the pouch $15. The three-item bundle costs $36 a $9 reduction from buying all three separately. For the notebook alone, choosing the bundle adds $16 to today’s payment.

Both calculations are correct. They answer different questions about what would otherwise have been bought.

Which set are you comparing?. Invented prices. The bundle saves $9 against all three items, but costs $16 more than the notebook alone.
Invented prices. The bundle saves $9 against all three items, but costs $16 more than the notebook alone. View larger.
Read the values
Which set are you comparing?
ItemValue
Notebook alone$20
Three-item bundle$36
All three separately$45

Shopify’s Buy X get Y documentation describes offers conditioned on particular quantities or spending. Eligibility can require adding items; it should be checked against the exact contents of the basket.

Compare versions and quantities too. A smaller notebook or a reduced pen pack makes the apparent comparison misleading if you use the separate price of a larger version. Returns may also have different conditions for the bundle.

If the other items replace purchases you already planned, the package may make sense. Write those items into the comparison instead of assuming every included part has value because it is offered at a discount.

Compare the total for the items you would use. If you only want the notebook, buying it alone leaves $16 more in your account.

Sources

Shopify: Buy X get Y discountsMerchant documentation of quantity and spending conditions.