We've asked in email to agree that an extended trial will cost X USD or that for their billing terms they need to agree to annual pre-payment. It's less formal but with enterprise B2B every sale is a big manual (and hopefully financially worth it). Generally we get a quick 'ok' or find another person (decision maker) added to cc in the reply. All the paperwork and billing comes far later than the trial, we have no problem moving that to the second month of the subscription. Trials with our service often take weeks. If a prospective client doesn't keep part of the deal, e.g. not happy with the trial, we don't charge. LOI (or one sentence in an email) is just a tactic to force a decision or figure out of the opposite party is able to make a decision. I wouldn't overthink every deal, you win some, you loose some and sometimes the company stops replying to emails for no reason.