Usps base price fallback

Do to nuances in how First Class mail pricing is returned, we need to fall back to commercial base pricing if it happens to cost less than commercial plus:

```
        <Postage CLASSID="61">
            <MailService>First-Class&amp;lt;sup&amp;gt;&amp;#8482;&amp;lt;/sup&amp;gt; Package Service</MailService>
            <Rate>0.00</Rate>
            <CommercialRate>2.76</CommercialRate>
            <CommercialPlusRate>4.05</CommercialPlusRate>
        </Postage>
```

cc @RichardBlair @snormore 
Instead of adding it to this test, can you create one with a title that explains your intension?

`Fallback to commercial rates when first class is less than commercial` or something like that?