German firm SumUp has announced a “double digit million Euro” investment round from American Express, Groupon and existing investors, as it hopes to mark itself out from rivals in Europe’s mobile point-of-sale market.

The firm said it will use its investment to support its expansion into new countries. Its service has been available in the UK, Germany, Ireland and Austria since August 2012. Since then it has launched in the Netherlands, Spain, Italy, France, Portugal and Belgium. And last week it launched in Russia.

Part of the funds will be spent on developing new services. The company promises to “further revolutionise the point-of-sale” as well as “build a point-of-sale ecosystem that connects merchants and consumers”.

Elsewhere, US peer Square has attempted to broaden its offering, rather than be dependent on the the mobile point-of-sale sector which is highly competitive.

Amex has been quieter on the mobile payments front than rivals MasterCard and Visa. Back in 2011, Amex executive vice president David Messenger said the credit card giant was looking for acquisitions, which he put into two categories: Significant investments outside the US and promising start-ups within the States.

Although not exactly an acquisition (more a start-up opportunity), SumUp does not fit either category.

Previously, in 2011, Amex acquired Sometrics, a virtual currency and in-games payments provider. It also participated in a funding round for Payfone, a carrier billing vendor, in 2011.

Meanwhile an investment by Groupon is not a surprise since the company has expressed its interest in the mobile payments market. However, the daily deals firm has previously launched its own card reader for the iPhone and iPod touch which begs the question why it would invest in a firm producing a rival product.

The current investment round follows a “double-digit million Euro” round last year from b-to-v Partners, Shortcut Ventures, Tengelmann Ventures, as well as business angels.  All those investors joined the latest round.

SumUP CEO Daniel Klein (pictured) described Amex as being “at the forefront of driving innovation in payments”. And Groupon, he said, “has proven that it is possible to build a company serving millions of small merchants worldwide in a short period of time”.