Estonia VAT act sets the mandatory data for sales invoices. Here is a sample sales invoice generated with Envoice online software.
If you are registered for VAT you need to display your VAT number.
If you are not registered for VAT you can add a note about not being registered for VAT. For example "Ei ole käibemaksukohustuslane" in Estonian or "Not registered for VAT" in English.
More specifically, Estonian VAT law § 37 prescribes what information needs to be on the invoices for them to be valid. VAT law applies to Estonian entities even if they are not yet registered for VAT in Estonia.
Some of the required fields for example are the serial number and date of issue of the invoice; the name and address of the issuer with VAT number (if registered for VAT); the name and address of the acquirer of goods or the recipient of services; the VAT number of the acquirer of goods or the recipient of services (if registered for VAT); the name or a description of the goods or services; the quantity of the goods or extent of the services, and the list continues with some more specific requirements such as references to reversed or zero-rated VAT clauses for cross-border sales.