I have the same problem. WordPress 4.3.1 and Amazon Post Purchase 2.0.0.
Line 81 in APP_Admin.class.php
$opts = get_option(AMZNPP_PLUGIN_OPTTIONS)[options];
and line 21 in APP_Widget.class.php
$this->options = get_option(AMZNPP_PLUGIN_OPTTIONS)[options];
Both throw mentioned parse error because of the [options]. I’m not sure if the syntax somehow changed in the last WP versions, but get_option(AMZNPP_PLUGIN_OPTTIONS)[options] seems to not be valid anymore.