I was having such a huge panic, freaking out that we would be ready to purchase a house and suddenly this $250 bill is haunting me forever lol. Bit dramatic
Even if this did end up on your credit rating, often if you pay the due amount and contact the creditor (explaining one off, etc), they will wipe it off your credit rating, letting you apply for loans, etc. However, others, don't count on creditors actually doing this, as far as I'm aware, they don't *need* to.

Another note for others; If you can't pay the total due amount in the seven days (or whatever the timeframe is the collection agency gives you), try to negotiate a payment plan (affordable to you), and work on getting it paid. It can be really hard to say "no, I'm sorry, I can't pay it all at once, can I pay it off, please?", but it's harder dealing with what the outcome could be if it is ignored repeatedly (ie, phone calls, letters, summons to court, ending in default listings on your credit rating). Trust me, I am, no, used to be, terrific at putting off due payments, etc, so take it from me, it is FAR EASIER to work out a plan (and then stick to it).

Sloane; often they will send a few letters; the first letter, a reminder (or two), and a final. Others; don't count on there being more than one letter, deal with it.