Documentation & Setup Guide

GMC Margin Tagger calculates gross profit margins for every active product in your Shopify catalog and generates standardized product tags. By mapping these tags into Google Merchant Center (GMC) Custom Labels, you can bid aggressively on high-margin winners and safeguard ad spend on low-margin inventory.

Gross Margin (%) = [ (Price - Cost) / Price ] × 100

1. Standard Generated Tags

Based on your configured thresholds, the app automatically generates and maintains the following tags:

Generated Tag	Default Threshold	Recommended Google Ads Strategy
gmc_high_margin	Margin ≥ 50%	Bid aggressively with lower target ROAS to maximize sales volume.
gmc_mid_margin	20% ≤ Margin < 50%	Balanced target ROAS with moderate volume scaling.
gmc_low_margin	Margin < 20%	High target ROAS or excluded from Performance Max bidding.
gmc_unknown_margin	Cost is 0 or Missing	Excluded from ad spend until cost data is entered in Shopify.

2. Step-by-Step Google Merchant Center Setup

Step 1: Synchronize Catalog in Shopify

Open GMC Margin Tagger from your Shopify Admin.
Review your profit margins and adjust threshold percentages if needed.
Check the confirmation box and click Save Sync.
Confirm tags appear under Products > Organization > Tags in Shopify.

Step 2: Access Feed Rules in GMC

Log in to your Google Merchant Center account.
Navigate to Products > Feeds (or Data Sources).
Click on your primary product feed (e.g., Content API from the Shopify Google & YouTube app).
Select the Feed Rules tab and click + Add Feed Rule.
Search for and select Custom label 0 (custom_label_0).

Step 3: Define Custom Label Mapping Rules

Configure conditional feed rules in Google Merchant Center to read your Shopify tags:

Rule 1 — High Margin

Condition: If tags contains gmc_high_margin
Set to: High Margin

Rule 2 — Medium Margin

Condition: If tags contains gmc_mid_margin
Set to: Medium Margin

Rule 3 — Low Margin

Condition: If tags contains gmc_low_margin
Set to: Low Margin

Rule 4 — Fallback / Unknown Cost

Condition: If tags contains gmc_unknown_margin (or fallback)
Set to: No Cost Data

Click Save as Draft, run Test Changes, and click Apply.

Step 4: Segment Campaigns in Google Ads

Open Google Ads and navigate to your Performance Max or Standard Shopping campaign.
Open Listing Groups (or Asset Groups > Listing Groups).
Click Subdivide and select Custom Label 0.
Assign custom target ROAS values or exclude low-margin products according to profitability.

Pro-Tip for Performance Max Excluding products tagged with No Cost Data prevents Google AI from bidding on items with unverified unit margins.

3. Frequently Asked Questions (FAQ)

How are multi-variant products handled?

Use the Variant Criterion selector in the app settings:
• Lowest Margin (Recommended): Tags the product based on the least profitable variant to avoid overspending on mixed items.
• Average Margin: Computes the arithmetic mean margin across variants.
• Highest Margin: Tags the product based on its top-performing variant.

Does this app modify my online store theme or slow down page speed?

No. GMC Margin Tagger runs completely in the Shopify Admin. It injects zero scripts, blocks, or snippets into your storefront theme and does not impact storefront loading speeds.

How quickly do tag updates reflect in Google Ads?

Once you click Save Sync, changes update in Shopify instantly. Google Merchant Center retrieves updated tags during its next feed fetch (typically within 1 to 24 hours depending on your feed configuration).

4. Support & API Compliance

Technical Assistance & Inquiries:

Email: support@astrolabs.io

Response Time: Within 24–48 business hours (Monday to Friday)

Built with Shopify App Bridge v4 & API version 2026-07. Fully compliant with GDPR data minimization protocols.