Just an example how to refactor and separate code and their concerns.

after_refactor_order_service.rb

class OrderService

  class << self
    def find_or_create_purchase_for_order(order, credit_card_id, shipping_fee = 0.0)
      user, vendor = order.buyer, order.seller
      purchase     = Purchase.find_by_user_id_and_order_id(user.id, order.id)

      return purchase if purchase.present?
      create_purchase order, credit_card_id, shipping_fee
    end

    def create_purchase(order, credit_card_id, shipping_fee)
      price, total = calculate_price order

      order.buyer.purchases.create(
          seller_id:           vendor.id,
          order_id:            order.id,
          shipping_address_id: order.cart.shipping_address.id,
          purchase_price:      price,
          sales_tax:           sales_tax,
          shipping_fee:        shipping_fee,
          credit_card_id:      credit_card_id,
          total_cost:          total,
          for_pickup:          true
      )
    end

    def calculate_price(order)
      vendor, sales_tax, price = order.seller, find_sales_tax(order), order.price

      total = (price + price * sales_tax) - vendor.reppio_cut
      [price, total]
    end

    def find_sales_tax(order)
      user, vendor = order.buyer, order.seller
      sales_tax    = SalesTax.find_by_user_id(vendor.id)

      sales_tax.send(
          if (addr = order.cart.shipping_address).present?
            addr.state.upcase.to_sym
          else
            user.state.upcase.to_sym
          end
      ).to_f rescue 0.0
    end
  end
end

before_refactor_order_service.rb

class OrderService
  def self.find_or_create_purchase_for_order(order, credit_card_id, shipping_fee = 0.0)
    user     = order.buyer
    purchase = Purchase.find_by_user_id_and_order_id(user.id, order.id)
    vendor   = order.seller
    unless purchase
      if order.cart.shipping_address
        shipping_address = order.cart.shipping_address
        sales_tax_row    = SalesTax.find_by_user_id(vendor.id)
        sales_tax = eval("sales_tax_row." + shipping_address.state.upcase).to_f rescue 0.0
      end

      sales_tax_row = SalesTax.find_by_user_id(vendor.id)
      begin
        sales_tax = eval("sales_tax_row." + user.state.upcase).to_f rescue 0.0000
      rescue
        sales_tax = 0.0000
      end
      price = order.price
      total = (price+ price*sales_tax) - vendor.reppio_cut

      purchase = user.purchases.build(
          :seller_id           => vendor.id,
          :order_id            => order.id,
          :shipping_address_id => order.cart.shipping_address.id,
          :purchase_price      => price,
          :sales_tax           => sales_tax,
          :shipping_fee        => shipping_fee,
          :credit_card_id      => credit_card_id,
          :total_cost          => total,
          :for_pickup          => true
      )
      purchase.save
    end

    purchase
  end
end

Here goes the other method, shortened and sweetened:

def find_sales_tax(order)
    sales_tax    = SalesTax.find_by_user_id(vendor.id)
    state = order.cart.shipping_address.try(:state) || order.buyer.state
    state ? sales_tax.send(state.to_sym).to_f : 0.0
end

hi @smsohan bhai, I LOVE this version! :)