margin was $3.0 million and $4.5 million for the three months ended June 30, 2020 and 2019, respectively, a decrease of $1.5 million, or 33%, primarily due to a decrease in bunkering activity.
Our commercial sales were $396.4 million and $692.4 million for the six months ended June 30, 2020 and 2019, respectively, a decrease of $296.0 million, or 43%, due to decreases in prices and volume sold. Our commercial product margin was $8.9 million and $11.0 million for the six months ended June 30, 2020 and 2019, respectively, a decrease of $2.1 million, or 19%, primarily due to a decrease in bunkering activity.
Selling, General and Administrative Expenses
SG&A expenses were $59.0 million and $41.0 million for the three months ended June 30, 2020 and 2019, respectively, an increase of $18.0 million, or 44%, including increases of $14.8 million in accrued discretionary incentive compensation, $1.2 million in professional fees, $0.9 million in costs associated with the COVID-19 pandemic, $0.7 million in wages and benefits and $0.4 million in various other SG&A expenses.
SG&A expenses were $99.9 million and $82.1 million for the six months ended June 30, 2020 and 2019, respectively, an increase of $17.8 million, or 22%, including increases of $12.3 million in accrued discretionary incentive compensation, $2.0 million in wages and benefits, $1.0 million in costs associated with the COVID-19 pandemic, $0.8 million in advertising costs, $0.7 million in professional fees and $1.0 million in various other SG&A expenses.
Operating Expenses
Operating expenses were $76.7 million and $86.4 million for the three months ended June 30, 2020 and 2019, respectively, a decrease of $9.7 million, or 11%, including decreases of $8.2 million associated with our GDSO operations, primarily due to lower credit card fees and maintenance and repair expenses and to the sale of sites, and $1.5 million associated with our terminal operations, primarily due to lower maintenance and repair expenses.
Operating expenses were $159.3 million and $169.4 million for the six months ended June 30, 2020 and 2019, respectively, a decrease of $10.1 million, or 6%, including decreases of $8.3 million associated with our GDSO operations, primarily due to lower credit card fees and maintenance and repair expenses and to the sale of sites, and $1.8 million associated with our terminal operations, primarily due to lower maintenance and repair expenses.
Lease Exit and Termination Gain
During the six months ended June 30, 2019, we were released from certain of our remaining obligations to provide future railcar storage, freight, insurance and other services for railcars under a fleet management services agreement associated with our 2016 voluntary termination of a railcar sublease. The release of certain obligations resulted in a $0.5 million reduction of the remaining accrued incremental costs, which benefit is included in lease exit and termination gain in the accompanying statement of operations for the six months ended June 30, 2019.
Amortization Expense
Amortization expense related to intangible assets was $2.7 million and $2.9 million for the three months ended June 30, 2020 and 2019, respectively, and $5.4 million and $5.9 million for the six months ended June 30, 2020 and 2019, respectively.
Net Gain on Sale and Disposition of Assets
Net gain on sale and disposition of assets was $0.8 million and $1.1 million for the three months ended June 30, 2020 and 2019, respectively, and $0.1 million and $0.6 million for the six months ended June 30, 2020 and 2019, respectively, primarily due to the sale of GDSO sites.