When switching from a bank to a credit union for an HSA.

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The old bank had the following text on the `DEPOSIT SLIP`, which was provided in the envelope together with their VISA debit card:

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> Prior year contributions must be postmarked between January 1 and the applicable Federal tax return deadline (typically April 15).\*
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> \*This does not include extensions.
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The new credit union required HSA account opening documents to be notarised by a local notary public, even though I've already have had several deposit accounts with them, and they do have an online banking web-site, supposedly capable of opening new account (but not "tax-advantaged accounts", according to them).

I notarised the application to open the new account on April 15, and mailed them back their BUSINESS REPLY MAIL packet the same day, prior to the last collection time at USPS, to make sure it was postmarked April 15, too.

However, they refused to make a prior year contribution, claiming my paperwork did not arrive until April 20.

Are they allowed to ignore the postmark date?