diff --git a/tests/utils/payment.test.ts b/tests/utils/payment.test.ts
index 18c2e97..7f68314 100644
--- a/tests/utils/payment.test.ts
+++ b/tests/utils/payment.test.ts
@@ -11,6 +11,7 @@ import {
   GAS_LIMIT_CAP,
   approvalAmountFor,
   boundedGasLimit,
+  merkleMaxCharge,
   payForQuotes,
   type RawPayment,
 } from '~/utils/payment'
@@ -39,6 +40,36 @@ describe('payment', () => {
     })
   })
 
+  describe('merkleMaxCharge', () => {
+    // 16 candidates like the contract's fixed pool size, amounts 1..16.
+    const pool = (amounts: number[]) => ({
+      candidates: amounts.map(a => ({ amount: BigInt(a) })),
+    })
+    const AMOUNTS = Array.from({ length: 16 }, (_, i) => i + 1)
+
+    it('mirrors the contract: median (index 8 of sorted 16) × 2^depth', () => {
+      // sorted[8] = 9; depth 7 → 9 × 128 = 1152
+      expect(merkleMaxCharge(7, [pool(AMOUNTS)])).toBe(1152n)
+    })
+
+    it('exceeds the old max-pool-sum bound for deep trees (the 0xfb8f41b2 regression)', () => {
+      // Pool sum is 136 — the allowance the old code verified. The vault
+      // actually pulls 1152 at depth 7; anything between reverted on-chain.
+      const sum = AMOUNTS.reduce((a, b) => a + b, 0)
+      expect(merkleMaxCharge(7, [pool(AMOUNTS)])).toBeGreaterThan(BigInt(sum))
+    })
+
+    it('takes the worst case across pools', () => {
+      const cheap = pool(AMOUNTS)
+      const dear = pool(AMOUNTS.map(a => a * 100))
+      expect(merkleMaxCharge(5, [cheap, dear])).toBe(900n * 32n)
+    })
+
+    it('handles empty pools without throwing', () => {
+      expect(merkleMaxCharge(7, [{ candidates: [] }])).toBe(0n)
+    })
+  })
+
   describe('boundedGasLimit', () => {
     it('adds 50% headroom to the estimate', () => {
       expect(boundedGasLimit(2_000_000n)).toBe(3_000_000n)
diff --git a/utils/payment.ts b/utils/payment.ts
index 8d5224c..9b9cb0a 100644
--- a/utils/payment.ts
+++ b/utils/payment.ts
@@ -234,6 +234,31 @@ export async function payForQuotes(
  * Pay for merkle tree via the PaymentVault contract (merkle path).
  * Single transaction for all chunks — lower gas for large uploads.
  */
+/** Upper bound of what PaymentVaultV2.payForMerkleTree will actually pull:
+ *  the vault charges `median(winner pool's 16 candidate amounts) × 2^depth`
+ *  (median16 = element 8 of the sorted 16), and picks the winner pool
+ *  on-chain from sender+timestamp — unknowable here, so take the worst
+ *  case across pools. The previous bound — the largest pool's SUM — is
+ *  ≈16×median, which UNDERSHOOTS the real charge by 2^depth/16 for
+ *  depth ≥ 5 (8× at depth 7). Wallets whose remaining allowance fell in
+ *  that gap skipped the approve and then reverted every retry with
+ *  ERC20InsufficientAllowance (0xfb8f41b2). */
+export function merkleMaxCharge(
+  depth: number,
+  pools: { candidates: { amount: bigint }[] }[],
+): bigint {
+  return pools.reduce((max, pool) => {
+    const amounts = pool.candidates
+      .map(c => c.amount)
+      .sort((a, b) => (a < b ? -1 : a > b ? 1 : 0))
+    if (amounts.length === 0) return max
+    // Contract's median16 selects index 8 of its fixed 16-slot array.
+    const median = amounts[Math.min(8, amounts.length - 1)]!
+    const charge = median * (1n << BigInt(depth))
+    return charge > max ? charge : max
+  }, 0n)
+}
+
 export async function payForMerkleTree(
   wagmiConfig: any,
   depth: number,
@@ -250,12 +275,7 @@ export async function payForMerkleTree(
     })),
   }))
 
-  const maxPoolCost = commitments.reduce((max, pc) => {
-    const poolTotal = pc.candidates.reduce((sum, c) => sum + c.amount, 0n)
-    return poolTotal > max ? poolTotal : max
-  }, 0n)
-
-  let gasSpent = await ensureAllowance(wagmiConfig, maxPoolCost)
+  let gasSpent = await ensureAllowance(wagmiConfig, merkleMaxCharge(depth, commitments))
 
   const call = {
     abi: paymentVaultAbi,