In this case, Infrastructure Cost is part of Cost of Revenue (similar to COGS or Cost of Goods Sold).
Cost of Revenue is defined on Page 61:
> Cost of revenue primarily consists of expenses related to providing our products to customers, including payments to our third-party cloud infrastructure providers for hosting our software, personnel-related expenses for operations and global support, including salaries, benefits, bonuses and stock-based compensation, payment processing fees, information technology, depreciation and amortization related to the amortization of acquired intangibles and internal-use software and other overhead costs such as allocated facilities.
> We intend to continue to invest additional resources in our platform infrastructure and our customer support and success organizations to expand the capability of our platform and ensure that our customers are realizing the full benefit of our platform and products. The level, timing and relative investment in our infrastructure could affect our cost of revenue in the future.
Cost of Revenue and infrastructure costs are discussed on Pages 64 and 65:
> Cost of revenue increased by $21.3 million, or 115%, for the six months ended June 30, 2019 compared to the six months ended June 30, 2018. This increase was primarily due to an increase of $17.9 million in third-party cloud infrastructure hosting and software costs, $1.0 million of depreciation and amortization, $1.6 million in personnel expenses as a result of increased headcount, $0.4 million of credit card processing fees and other fees, and $0.4 million in allocated overhead costs as a result of an increase in overall costs necessary to support the growth of the business and related infrastructure.
Tip: Search the S-1 for "infrastructure" or "cost of revenue" to find other mentions of this.