.htaccess mod重写导致远程服务器上的表单操作URL的奇怪错误

I am not sure if the problem is the .htaccess file but it is what my hosting company are telling me.

I have used a framework called Perch for a site I am building which I have tweaked to work how I want it to for a shopping cart area on the site. The site works fine on my local server (MAMP) but since I have uploaded to my remote server I am having a problem adding an item to the cart (using a straight forward form post action whilst testing and getting it to work).

My .htaccess file includes all of the following:

Options +FollowSymlinks
RewriteEngine On

RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteRule ^(.+)/?$ $1.php [L,QSA]

RewriteRule ^dog-agility-equipment/([a-zA-Z0-9-/]+)$ /dog-agility-equipment/category.php?cat=$1 [L]
RewriteRule ^agility-equipment/([a-zA-Z0-9-/]+)/([a-zA-Z0-9-/]+)$ /dog-agility-equipment/product.php?s=$2 [L]
RewriteRule ^agility-equipment/([a-zA-Z0-9-/]+)$ /dog-agility-equipment/product.php?s=$1 [L]

The form action [url] is

/manage/addons/apps/perch_shop_paypal/cart.functions.php

And the server is trying to direct to the url

http://site-name.uk/manage/?r=%2Fmanage%2Faddons%2Fapps%2Fperch_shop_paypal%2Fcart.functions.php

I have never seen this before and can't see why my .htaccess file would be causing this issue (especially as it doesn't happen on my local server).

Does anyone know why this is happening, what am I missing?