As a professional triathlete, you are your own sales, marketing, and human resources department. You do not have a team who invoices your partners for salary payments, podium incentives, and gear. This is something you have to keep track of and send to your sponsors in a timely manner so you can receive proper payment.
Create an Excel spreadsheet with your sponsors and corresponding payment schedule and use this invoice to bill them at the appropriate times. You can adjust this invoice to include your bank information if you work it out to be paid via wire instead of check. Once you have filled out the invoice, make it into a PDF before sending so it cannot be adjusted as it finds its way to the billing department.