```ruby
require 'spec_helper'

describe Spree::Calculator::PriceSack, type: :model do
  let(:calculator) do
    calculator = Spree::Calculator::PriceSack.new
    calculator.preferred_minimal_amount = 5
    calculator.preferred_normal_amount = 10
    calculator.preferred_discount_amount = 1
    calculator
  end

  let(:order) { stub_model(Spree::Order) }
  let(:shipment) { stub_model(Spree::Shipment, amount: 10) }

  # Regression test for https://github.com/spree/spree/issues/714 and https://github.com/spree/spree/issues/739
  it "computes with an order object" do
    calculator.compute(order)
  end

  # Regression test for https://github.com/spree/spree/issues/1156
  it "computes with a shipment object" do
    calculator.compute(shipment)
  end

  # Regression test for https://github.com/spree/spree/issues/2055
  it "computes the correct amount" do
    expect(calculator.compute(2)).to eq(calculator.preferred_normal_amount)
    expect(calculator.compute(6)).to eq(calculator.preferred_discount_amount)
  end
end
```

Change model tested to correct model
user
```ruby
require 'spec_helper'

describe Spree::Calculator::Shipping::PriceSack, type: :model do
  let(:calculator) do
    calculator = Spree::Calculator::PriceSack.new
    calculator.preferred_minimal_amount = 5
    calculator.preferred_normal_amount = 10
    calculator.preferred_discount_amount = 1
    calculator
  end

  let(:order) { stub_model(Spree::Order) }
  let(:shipment) { stub_model(Spree::Shipment, amount: 10) }

  # Regression test for https://github.com/spree/spree/issues/714 and https://github.com/spree/spree/issues/739
  it "computes with an order object" do
    calculator.compute(order)
  end

  # Regression test for https://github.com/spree/spree/issues/1156
  it "computes with a shipment object" do
    calculator.compute(shipment)
  end

  # Regression test for https://github.com/spree/spree/issues/2055
  it "computes the correct amount" do
    expect(calculator.compute(2)).to eq(calculator.preferred_normal_amount)
    expect(calculator.compute(6)).to eq(calculator.preferred_discount_amount)
  end
end
```
assistant