Most member states accept a sponsor, and several have an official undertaking document you complete at your town hall or online rather than a letter you write. Which one applies depends on the country deciding the application.
Germany uses a formal declaration of commitment, obtained from the foreigners authority. The Netherlands and Belgium have their own forms. France has an attestation d'accueil from the mairie. Where the state has a form, a letter you wrote instead of it does not replace it.
Your bank statements, your payslips or income evidence, proof of your residence status, and proof of the address where your visitor will stay. The consulate is testing whether the means exist, and that is an arithmetic question about your documents.
Your contribution is set against the member state's published daily figure multiplied by the days requested. If you are covering accommodation but not living costs, say exactly that, so the consulate can do the sum rather than guess.
Open the case as the sponsor, add your own documents, and share the link with your relative. Approvable drafts the invitation and undertaking, and tells you where a member state form has to be used instead.
You can open the case, pay for it and add your own documents, then send the link to your relative so they can add theirs. One purchase covers the case whoever pays.
Our software reads your documents, checks every ground, tells you exactly what is thin and what would fix it, and writes the cover letter, the sponsor letter and the employer letter from the facts you gave. One re-run included within 30 days.