Breakdown of El proveedor envió el presupuesto.
Questions & Answers about El proveedor envió el presupuesto.
Because proveedor and presupuesto are both singular masculine nouns, and here they refer to specific things: a particular supplier and a particular estimate/quote.
Spanish usually keeps the definite article in cases where English might sometimes drop it. If the meaning were a supplier sent a quote, you would normally say Un proveedor envió un presupuesto.