If the Patent Office decides your invention is genuinely new and you're therefore entitled to a patent under the law, they'll send you a notice of allowance. The notice will specify how much the issue fee will be; if you don't pay within 3 months--no extension allowed--your application will be voided. If the Patent Office intends to publish your patent application, something that happens within 18 months of applying unless you fall under the exceptions in federal regulations, the notice of allowance will also tell you the publication fee, which must also be paid within 3 months, no extensions.
