ended September 30, 2020 due to additional overhead costs. Professional legal and accounting fees increased for the nine months ended September 30, 2021 to $256,732 as compared to $61,512 for the nine months ended September 30, 2020 as a result of increased lease review and business operations. Software fees increased for the nine months ended September 30, 2021 to $211,335 as compared to $121,826 for the nine months ended September 30, 2020 as a result of increased unit counts resulting in additional revenue management and sales distribution software needs.
Total Other (Expense)
Total other (expense), for the nine months ended September 30, 2021 was $(1,226,328) as compared to ($451,860) for the nine months ended September 30, 2020. These expenses are due to interest and financing costs related to borrowings for working capital.
Year Ended December 31, 2020 as compared to Year Ended December 31, 2019
Rental Revenue
The increase in net rental revenue of 52% for the year ended December 31, 2020 to $8,273,859 as compared to $5,433,544 for the year ended December 31, 2019 was a result of the increase in units available to rent from 137 at December 31, 2019 to 330 at December 31, 2020.
Cost of Revenue
For the year ended December 31, 2020, the principal component responsible for the increase in our cost of revenue was rental expenses for our units available to rent, which increased by $4,056,628, or 181%, from $2,235,676 in the year ended December 31, 2019, as a result of the increase in size of our rental unit’s portfolio period over period. Our increased rental expenses were offset, in part, by favorable rent concessions received from our landlords in 2020 as a result of COVID. As a result of the increase in size of our rental units portfolio period over period, we had other expense, including contracted services fees, which increased by approximately $739,139, or 291%, during the year ended December 31, 2020, as compared to $253,262 for the year ended December 31, 2019, furniture rental fees, which increased by approximately $458,838, or 164%, during the year ended December 31, 2020, as compared to $279,588 for the year ended December 31, 2019, commissions and booking expenses, which increased by approximately $655,452, or 101% during the year ended December 31, 2020, as compared to $647,288 for the year ended December 31, 2019, and cleaning fees, which increased by approximately $262,488, or 38%, as compared to $693,563 for the year ended December 31, 2019. Additionally, we had material increases in processing fees period over period of approximately, $208,930, or 341%. Our processing fees increased due to higher processing volumes. Cable, WIFI and utilities fees also increased by approximately $157,170, or 119%, period over period.
Gross Profit (Loss)
The decrease in our gross profit margins of $3,681,373 to ($2,958,784) or approximately 509% for the year ended December 31, 2020 as compared to $722,589 for the year ended December 31, 2019 is primarily attributable to Covid-19 government related travel restrictions and shutdowns that reduced our occupancy rates.
Total Operating Costs
Total operating costs incurred for the year ended December 31, 2020 increased by approximately 34% to $1,107,240 as compared to $826,792 for the year ended December 31, 2019. Operating costs include contracted services, selling and administrative expenses, professional fees, and software fees. During the years ended December 31, 2020 and 2019, our largest operating cost was contracted services of $521,993 and 522,993, respectively, which represented approximately 47% and 63% of our operating costs, respectively. The increase in total operating costs from 2019 to 2020 is primarily attributable to the operation of 193 additional units as of December 31, 2020. Selling and administrative expenses increased for the year ended December 31, 2020 to $351,774 as compared to $156,392 for the year ended December 31, 2019 due to additional overhead costs. Professional legal and accounting fees increased for the year ended December 31, 2020 to $61,512 as compared to $12,500 for the year ended December 31, 2019 as a result of increased