Priority Mail Cubic: The Secret to Cheap Heavy Package Shipping

Priority Mail Cubic is one of USPS's best-kept secrets for saving money on shipping. Unlike standard Priority Mail that charges based on weight and distance, Cubic pricing is based purely on package dimensions—making it perfect for heavy items in small boxes.

What is Priority Mail Cubic?

Priority Mail Cubic is a special USPS pricing option exclusively available through commercial shipping platforms like AtoShip. It offers:

• Dimension-based pricing - Rates based on cubic size, not weight
• Up to 20 lbs included - No weight surcharge for heavy packages
• 2-3 day delivery - Same speed as standard Priority Mail
• $100 insurance included - Free coverage on every package
• Full tracking - Real-time updates to delivery

Why Cubic Pricing Saves Money

Traditional shipping calculates cost using both weight and dimensions. For heavy items in compact boxes, this often results in higher charges. Cubic pricing ignores weight entirely.

Example: Shipping a 15 lb item in a 10x8x6 inch box

Cubic Pricing Tiers

Priority Mail Cubic uses 5 pricing tiers based on package volume:

How to Calculate Cubic Size

For Boxes

Measure outer dimensions (length × width × height) in inches

Multiply all three dimensions together

Divide by 1,728 to convert to cubic feet

Round up to the next tier

Example:

• Box dimensions: 10 × 8 × 6 inches
• Volume: 10 × 8 × 6 = 480 cubic inches
• Cubic feet: 480 ÷ 1,728 = 0.278
• Tier: 0.3 (rounded up from 0.278)

For Softpacks (Poly Mailers/Padded Envelopes)

Measure length and width in inches

Add them together

Use the combined measurement to determine tier

Requirements and Restrictions

Package Requirements

• Maximum 0.5 cubic feet (864 cubic inches)
• Maximum 20 lbs weight
• Maximum 18 inches on any single side
• Minimum package dimensions apply

What You Get

• 2-3 business day delivery
• $100 free insurance
• Free tracking with delivery confirmation
• Saturday delivery at no extra charge

Best Products for Cubic Shipping

Priority Mail Cubic is ideal for:

• Candles and home goods - Dense, heavy items
• Books and media - Compact but weighty
• Beauty products - Small jars and bottles
• Electronics accessories - Cables, chargers, small devices
• Food items - Specialty foods, spices
• Jewelry - Small boxes, higher weight materials
• Hardware and tools - Dense metal items

How to Access Cubic Pricing

Priority Mail Cubic is NOT available at Post Office counters or through USPS.com directly. You must use a commercial shipping platform.

With AtoShip:

Create a free account

Enter package dimensions and weight

Cubic pricing automatically appears when eligible

Print your label and ship

No volume minimums, no monthly fees, no contracts.

Cubic vs. Flat Rate: Which is Better?

USPS Flat Rate boxes offer predictable pricing but aren't always the best deal.

Rule of thumb: For packages under 0.5 cubic feet, always compare Cubic pricing.

Pro Tips for Maximizing Cubic Savings

1. Right-Size Your Packaging

Use the smallest box that safely fits your item. Smaller = lower tier = lower cost.

2. Use Poly Mailers When Possible

Softpack cubic tiers are even more generous, allowing larger combined dimensions.

3. Calculate Before Shipping

Always check cubic eligibility. AtoShip automatically shows when Cubic pricing applies.

4. Combine Dense Items

If shipping multiple heavy items, combining them in one cubic-eligible package often costs less than shipping separately.

Start Saving with Cubic Pricing

Priority Mail Cubic represents one of the best values in shipping for e-commerce sellers dealing with dense, heavy products. By understanding the tier system and using a platform like AtoShip, you can access these rates immediately.

Sign up free and see how much you can save on your next shipment.