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 
The `.to_s` covers the nil case.