This is the sort of economics that small companies face when working with large companies, particularly when physical things/CAPEX are involved. Large companies expect net 30/60 terms to pay you. That's much simpler for their accounting/purchasing department. This bureaucracy occasionally necessitates nudging, especially if the intermediary you're dealing with didn't set up the invoice request on time in whichever SAP/Salesforce/Oracle system they use.

This is usually the same the other way; many vendors will give business clients net 30. That's nice if you're a small company and need to plan ahead. But occasionally, because you're considered small (liability), some vendors will want the money up-front. So unless you're very careful with cashflow, you end up in situations where your main sources of income (big contracts) are coming in after you need to spend money on a widget to fulfill deliverables.

Depending on the situation, the contract can demand the client purchase/ship things and work doesn't start until you have them in hand. This is usually the best route as you now have an out, and it's not an unreasonable request, but it doesn't always pan out that way!