How do I reserve inventory for prepaid subscription boxes so month 2 and 3 contents aren't stolen by DTC?

A prepaid 3-pack is one payment and three future picks. Shopify does not reserve month 2 and month 3 at checkout. It reserves prepaid inventory only when each cycle's fulfill date hits. Until then those units look sellable. DTC will take the coffee you bought for March.

Park future contents in a hold

Park the future contents now. A reserved quantity, a virtual zone outside the primary sellable zone, or a funded pool for the subscription channel. Remainder math then publishes DTC the leftover. A display pool that does not stop warehouse allocation will still let a 20-unit DTC wave take the 20 you showed as held for February. Physical hold, then pick the box against that hold. If you cannot name the zone or pool that owns March, DTC owns March.

Build a contents calendar by month

Build a contents calendar by ship month, not by checkout month. Month 1 may already sit in the forward pick face. Months 2 and 3 stay in reserve until their fulfill date. If the recipe changes, move the hold. Do not hope Shopify scheduled fulfillments will catch a new SKU. Many 3PL and ShipStation-class apps never import those scheduled fulfillments. If the 3PL only sees today's wave, they will not protect March. Tell the 3PL the reserved qty, or keep that stock in a zone they cannot pick for DTC.

Reserve by cycle, not one kit

Kits are a one-time assembly. A subscription box is a calendar of picks against one prepaid order. Do not kit three months of inventory onto one work order on day one unless you want that stock sitting assembled and still at risk if someone unkits. Reserve the components, or the finished inserts, by cycle. Receive the inbound that funded the subscription into that hold, or the remainder math lies and DTC sees a pile that was already sold. A PO bought "for the box" that receipts into the primary sellable zone is not reserved. It is a donation to the next drop.

One payment does not mean one inventory event. Three holds, three picks, three ship confirmations. This is not a revenue question. Cash at checkout is a different post (when the box is earned, not when the card is charged). Here the only job is to keep month 2 and month 3 off the DTC available-to-sell until their fulfill date. If Shopify is the only reserve, March is unprotected until that date arrives. Count prepaid open qty against the hold every week. If prepaid boxes outrun the reserved components, DTC is already eating the box, or the inbound never landed in the pool.