Our same-store tons sold decreased 5.0% and 5.5% in the three months and six months ended June 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 7.6% and 6.9%, respectively, during the same periods, and included one less shipping day in the 2019 six-month period. Our same-store average selling price per ton sold in the three months and six months ended June 30, 2019 increased 0.2% and 6.2%, respectively, from the comparable 2018 periods. Our investments in value-added processing equipment during the past several years and focus on specialty products supported our gross profit margins of 29.6% and 29.5% in the three months and six months ended June 30, 2019, respectively, exceeding our estimated sustainable range of 27% to 29%.
Our S,G&A expense as a percent of sales of 18.4% and 18.2% in the three months and six months ended June 30, 2019, respectively, changed from 17.9% and 18.4% in the comparable 2018 periods, respectively, due to changes in our sales levels as we continued to exercise effective expense control.
Due to our strong gross profit margin and effective working capital management, we generated significant cash flow from operations of $463.2 million in the six months ended June 30, 2019, up significantly from $97.0 million in the comparable 2018 period. As of June 30, 2019, our net debt-to-total capital ratio was 27.4%, down from 30.8% as of December 31, 2018.
We believe that our exposure to diverse end markets, a broad product base, just-in-time delivery, 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 differentiates us from our peers and has 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. We believe we have sufficient liquidity as of June 30, 2019, with $699.0 million available for borrowing on our revolving credit facility, $141.5 million in cash and cash equivalents, and access to capital to continue executing our capital allocation strategy.
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 $15.2 million for the six months ended June 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 $22.5 million for the six months ended June 30, 2019. We have consolidated the financial results of Acero Prime since October 1, 2014 when we acquired a controlling 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 precision sheet metal fabrication ranging from prototypes to large production runs which utilize a wide variety of metals and fabrication methods including laser cutting, stamping, turret punching, machining, powder coating and welding. KMS’ net sales were $18.0 million for the six months ended June 30, 2019.