In short: An invoice is a document proving the sale of goods or a service — what, when, between whom and at what price — showing VAT separately when the seller is VAT-registered.
Simply put: For the customer, an invoice proves the purchase — you need it for a complaint, a guarantee or a report. For the business, it is an accounting document. A VAT-registered business sees the tax as a separate line and may use it under the rules. In publicly funded projects every cost must be backed by a document.
Example: A firm buys a laptop. The invoice shows a base of €1,000, VAT at 20% — €200, total €1,200. The invoice and the guarantee card are kept together.
Watch out: the compulsory details of an invoice and the deadlines for issuing it are set in tax law — check them with the tax authority, don't write them from memory (*). A missing document in a project leads to a financial correction.
Example: Bulgaria — The standard VAT rate is 20% (VAT Act, Article 66). Not every business is VAT-registered — only those above the legal threshold (€51,130 annual turnover from 1 January 2026, Article 96(1)) or those that register voluntarily.
Check in your country — The EU sets common VAT rules (VAT Directive 2006/112/EC); the details of invoices and thresholds are applied nationally. Ask your tax authority.
Source: Council Directive 2006/112/EC — https://eur-lex.europa.eu/eli/dir/2006/112/oj · NRA, "VAT in Bulgaria" — https://nra.bg/wps/portal/nra/taxes/dds-v-balgariya (in Bulgarian)