Originally, it had "~/ucommerceapi" as the reserved path. I tried using "/ucommerceapi/" and "/ucommerceapi" there, but the api call is still forced to lowercase.

Could you share your fix? Because im having the same issue. Thanks

Sure, I did two things.

First, I disabled seo checker's url rewriting in /App_Plugins/SEOChecker/config/SEOChecker.config. This makes ucommerce work, but loses the general redirects that seo checker adds. The urlRewrite tag should be as follows.

<urlRewrite enableUrlRewriting="false" forcewwwUrls="Ignore" useTrailingSlashProperty="True" />

Settings "enableUrlRewriting" to false disables SEO Checker's general redirects.

Then I added a couple rules to the web config to reproduce a couple of the redirects that SEO Checker normally handles.

<rule name="Force lower case" stopProcessing="false">
  <match url="(.*)"/>
  <conditions>
    <add input="{R:1}" pattern="^umbraco|ucommerceapi|app_plugins|media" negate="true"/>
    <add input="{URL}" pattern="[A-Z]" ignoreCase="false" />
  </conditions>
  <action type="Redirect" url="{Tolower:{R:0}}" appendQueryString="true" redirectType="Permanent"/>
</rule>
<rule name="Force trailing slash" stopProcessing="false">
  <match url="(.*[^/])$"/>
  <conditions>
    <add input="{R:1}" pattern="^umbraco|ucommerceapi|app_plugins|media" negate="true"/>
    <add input="{REQUEST_FILENAME}" matchType="IsFile" negate="true" />
    <add input="{REQUEST_FILENAME}" pattern="(.*?)\.html$" negate="true" />
    <add input="{REQUEST_FILENAME}" pattern="(.*?)\.aspx$" negate="true" />
    <add input="{REQUEST_FILENAME}" pattern="(.*?)\.axd" negate="true" />
  </conditions>
  <action type="Redirect" url="{R:1}/" appendQueryString="true" redirectType="Permanent"/>
</rule>

You may need to tinker with the conditions a bit to get it to work in your case, but this it what I've done. It's not perfect, but better than nothing.