The amount paid for the property does not exceed $2500 per invoice (or per item as substantiated by the invoice) 

here's an example from the IRS regulations  - each item is less than $2500 but the total invoice over $2500

requirements to elect de minimis safe harbor if no applicable financial statements

(A) The taxpayer does not have an applicable financial statement (as defined in paragraph (f)(4) of this section);

(B) The taxpayer has at the beginning of the taxable year accounting procedures treating as an expense for non-tax purposes -

(1) Amounts paid for property costing less than a specified dollar amount; or

(2) Amounts paid for property with an economic useful life (as defined in § 1.162-3(c)(4)) of 12 months or less;

(C) The taxpayer treats the amount paid for the property as an expense on its books and records in accordance with these accounting procedures; and

(D) The amount paid for the property does not exceed $2500 per invoice (or per item as substantiated by the invoice) or other amount as identified in published guidance in the Federal Register or in the Internal Revenue Bulletin (see § 601.601(d)(2)(ii)(b) of this chapter).

Example. De minimis safe harbor; taxpayer without Applicable Financial Statement. (as revised per the latest IRS notice (changes $500 to $2500) 
In Year 1, B purchases 10 computers at $600 each for a total cost of $6,000 as indicated by the invoice. Assume that each computer is a unit of property under § 1.263(a)-3(e). B does not have an AFS. B has accounting procedures in place at the beginning of Year 1 to expense amounts paid for property costing less than $1,000 (up to $2,500) and B treats the amounts paid for the computers as an expense on its books and records. The amounts paid for the printers meet the requirements for the de minimis safe harbor under paragraph (f)(1)(ii) of this section because the amount paid for the property (while the invoice is for more than $2,500) is less than $1000 for each computer (substantiated by the invoice). B may apply the de minimis safe harbor election to the amounts paid for the 10 computers under paragraph (f)(1) of this section.

so what was your specified $ amount for expensing at the beginning of your tax year for accounting purposes 

see highlighted line above 

if you used less than $2000 then the windows would need to be expensed

part of the reg 1.263(a)-1 is an anti-abuse rule

(6) Anti-abuse rule. If a taxpayer acts to manipulate transactions with the intent to achieve a tax benefit or to avoid the application of the limitations provided under paragraphs (f)(1)(i)(B)(1), (f)(1)(i)(D), (f)(1)(ii)(B)(1), and (f)(1)(ii)(D) of this section, appropriate adjustments will be made to carry out the purposes of this section. For example, a taxpayer is deemed to act to manipulate transactions with an intent to avoid the purposes and requirements of this section if -

(i) The taxpayer applies the de minimis safe harbor to amounts substantiated with invoices created to componentize property that is generally acquired or produced by the taxpayer (or other taxpayers in the same or similar trade or business) as a single unit of tangible property; and

(ii) This property, if treated as a single unit, would exceed any of the limitations provided under paragraphs (f)(1)(i)(B)(1), (f)(1)(i)(D), (f)(1)(ii)(B)(1), and (f)(1)(ii)(D) of this section, as applicable.

note that (i) and (ii) are only examples,