Our same-store tons sold decreased 2.0% and 4.4% in the three months and nine months ended September 30, 2019, respectively, compared to the same periods in 2018, outpacing the industry data reported by the Metals Service Center Institute (“MSCI”), which indicated industry shipments were down 7.1% and 7.0%, respectively, during the same periods. Our investments in value-added processing equipment during the past several years and focus on specialty products supported our gross profit margins of 30.3% and 29.7% in the three months and nine months ended September 30, 2019, respectively, exceeding our estimated sustainable range of 27% to 29%.
Our S,G&A expense declined $12.3 million, or 2.3%, and $4.1 million, or 0.3%, in the three months and nine months ended September 30, 2019, respectively, compared to the same periods in 2018, due to continued effective expense control. However, our S,G&A expense as a percentage of sales of 19.3% and 18.6% in the three months and nine months ended September 30, 2019, respectively, increased from 17.9% and 18.2% in the comparable 2018 periods, respectively, due to decreases in our sales levels. The impact of our lower sales levels on the increase in our S,G&A expense as a percentage of sales was most pronounced in the three months ended September 30, 2019 compared to the same period in 2018 and was mainly due to lower metal pricing.
Strong operating income and effective working capital management generated significant cash flow from operations of $954.1 million in the nine months ended September 30, 2019, up significantly from $233.3 million in the comparable 2018 period. As of September 30, 2019, our net debt-to-total capital ratio was 22.6%, down from 30.8% as of December 31, 2018.
We believe that our broad end market exposure, diverse product offerings, focus on small order sizes, just-in-time delivery and significant value-added processing capabilities along with our wide geographic footprint will continue to mitigate earnings volatility compared to many of our competitors. We believe that these business model characteristics combined with pricing discipline and our strategy of concentrating on higher margin business differentiate us from our peers and have allowed us to achieve industry-leading results.
We will continue to focus on working capital management and maximizing profitability of our existing businesses, as well as executing our proven growth strategies and stockholder return activities. As of September 30, 2019, we had $1.05 billion available for borrowing on our revolving credit facility and $166.0 million in cash and cash equivalents. We believe our sources of liquidity will continue to be adequate to maintain operations, finance strategic initiatives, pay dividends, and execute purchases under our share repurchase program.
Acquisitions
2018 Acquisitions
On November 1, 2018, we acquired All Metals Holding, LLC, including its operating subsidiaries All Metals Processing & Logistics, Inc. and All Metals Transportation and Logistics, Inc. (collectively, “All Metals”). All Metals is headquartered in Spartanburg, South Carolina with an additional facility in Cartersville, Georgia. All Metals specializes in toll processing for automotive, construction, appliance and other diverse-end markets, and provides value-added transportation and logistics services for metal products from six strategically located terminals throughout the southeastern United States. All Metals’ net sales were $22.6 million for the nine months ended September 30, 2019.
On October 23, 2018, we purchased the remaining 40% noncontrolling interest of Acero Prime, S. de R.L. de C.V. (“Acero Prime”), a toll processor in Mexico, which increased our ownership from 60% to 100%. Acero Prime, headquartered in San Luis Potosi, Mexico, has four toll processing locations. Acero Prime performs metal processing services such as slitting, multi-blanking and oxy-fuel cutting, as well as storage and supply-chain management for a variety of different industries including automotive, home appliance, lighting, HVAC, machinery and heavy equipment. Acero Prime’s net sales were $34.3 million for the nine months ended September 30, 2019. We have consolidated the financial results of Acero Prime since October 1, 2014 when we acquired a majority interest.
On August 1, 2018, we acquired KMS FAB, LLC and KMS South, Inc. (collectively, “KMS” or the “KMS Companies”). The KMS Companies are headquartered in Luzerne, Pennsylvania. The KMS Companies specialize in