Ok got it. In that case it makes for a tough situation. You should always send things to USCIS in the future at the bare minimum with priority flat rate mailing, which has free delivery confirmation for about $5. Because you sent it with stamps this time, there will be no way to know for sure if it made it or if it's being returned for whatever reason. If it's being returned, it could take some time.
In any case, first it's important to remember that a receipt notice usually takes about 2 weeks. Given that, it's possible that you may get it at the end of the week. However, I know that her status expires this week. You have two options as I see it- tomorrow you call USCIS at 1-800375-5283 and make sure they have no record of it. IF they do not have record and the check is still not cashed, it may be better to put a stop payment on the check and resend the entire package again this time through express priority mail (arrives overnight) to make sure that you will have a record of it arriving before her status expires on the 3rd. If they do have a record of it or the check is cashed then obviously you don't have to do it and you are fine, even if the receipt notice arrived after the 3rd.
The second choice is just to wait a little longer and see if the receipt notice expires even after her status expires. If it doesn't arrive in another week or two, then put a stop payment and resend as I stated above. Yes, she will technically be out of status for a short period, but it's fine because as soon as USCIS receives the new application, she will be a temporary legal status again while the application is pending and she'll be able to get her green card the same, without an issue.
Good luck to you and I hope this answer has helped you!
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